12v lithium batteries can't be paralleled?

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12v lithium batteries can't be paralleled?

Post by Paul9 »

Hi people,

My brother wants to buy 2 or 3 12v lithium batteries about 100ah each with internal BMS for his camping trips. We were hunting around for suppliers and found a brand called Voltax but the website says they can't be used in parallel only in series. We found other brands but they do not mention this restriction.

Could someone clear me up on this please?
Thanks
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Re: 12v lithium batteries can't be paralleled?

Post by brendon_m »

Normally it's the other way, ie they can't be used in series
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Re: 12v lithium batteries can't be paralleled?

Post by jonescg »

I think the BMS might not like the cascade in the event of a low or high battery. If one BMS goes open circuit, the others are still closed, but the load on them increases by a third. Until one of those goes open circuit, and the whole load is being borne by the one remaining. And then it goes.

Presumably :?
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Re: 12v lithium batteries can't be paralleled?

Post by Paul9 »

I just checked and it definitely says they can't be paralleled but it is ok to link two in series to give you 24 v.

Thanks guys for your replies.
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Re: 12v lithium batteries can't be paralleled?

Post by dRdoS7 »

Hi,
jonescg wrote: Fri, 08 May 2020, 17:21 I think the BMS might not like the cascade in the event of a low or high battery. If one BMS goes open circuit, the others are still closed, but the load on them increases by a third. Until one of those goes open circuit, and the whole load is being borne by the one remaining. And then it goes.

Presumably :?
As long as the user stays within the limits of the battery's BMS, i.e. Max. current draw, it shouldn't be a problem. If it is, then the batteries aren't up to spec.

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Re: 12v lithium batteries can't be paralleled?

Post by brendon_m »

The voltax batteries are pretty cheap ($580 for 100Ah) which made my spider sense tingle.
The specs are rated to 50A max (20A recommended). Ok for running light loads like LED lights for long periods but don't expect an inverter to run off it.
That's no doubt a limitation of the cheap bms that they have used but possibly the cells as well.

Not quite as cheap ($875 for 120Ah) as those voltax ones but pretty close and these can be put in parallel and series up to 24v and are good for 300A
https://www.ev-power.com.au/product/bat-evh12v120ah/
(website says no stock but everything look it says that, so maybe the website doesn't get updated much)
And they are from someone I trust.
Last edited by brendon_m on Sun, 10 May 2020, 09:09, edited 1 time in total.
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Re: 12v lithium batteries can't be paralleled?

Post by Paul9 »

Thanks guys

Advice appreciated!

Paul
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

Lithium batteries inherently have a very low internal resistance. if you attempt to parallel multiple 12v batteries, one battery will do all the work until it fails and that will kill the other batteries in parallel. This also goes for the maximum current load and continuous current load, they will not share that load over multiple parallel batteries, so if the limit for one battery is 50 amps, no matter how many batteries you put in parallel the limit is still 50 amps. If the BMS in the battery doing all the work fails, the next in line as far as resistance gets all the load until it fails and so on. A warranty claim will be rejected because the BMS capacity was exceeded.
We do get a bit of work from making these multiple 12v batteries sort of work together after one battery in the group fails, the supplier replaces it after a fight but refuses to replace any more if they fail claiming it must be the way you installed them if they keep failing.
Recharging is the next problem, the battery with the least resistance will charge first, when it reaches the end of charge voltage the charger drops into float. To get the battery with the longest leads and highest resistance to fully charge, the battery with the lowest resistance must have its terminal voltage climb way higher than is suitable for extended battery life.
Just because the voltage is the same across all the batteries in parallel doesn't mean they will all remain at a common state of charge, one will remain virtually fully charged while one gets worked very hard because the voltage will not drop until the weakest cell is fully discharged, well beyond the 0% SOC. In severe situations the weakest cell will suffer with reverse current flow destroying the cell in a matter of minutes.

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Re: 12v lithium batteries can't be paralleled?

Post by dRdoS7 »

Hi,
T1 Terry wrote: Tue, 12 May 2020, 13:40 Lithium batteries inherently have a very low internal resistance. if you attempt to parallel multiple 12v batteries, one battery will do all the work until it fails and that will kill the other batteries in parallel. This also goes for the maximum current load and continuous current load, they will not share that load over multiple parallel batteries, so if the limit for one battery is 50 amps, no matter how many batteries you put in parallel the limit is still 50 amps. If the BMS in the battery doing all the work fails, the next in line as far as resistance gets all the load until it fails and so on. A warranty claim will be rejected because the BMS capacity was exceeded.
We do get a bit of work from making these multiple 12v batteries sort of work together after one battery in the group fails, the supplier replaces it after a fight but refuses to replace any more if they fail claiming it must be the way you installed them if they keep failing.
Recharging is the next problem, the battery with the least resistance will charge first, when it reaches the end of charge voltage the charger drops into float. To get the battery with the longest leads and highest resistance to fully charge, the battery with the lowest resistance must have its terminal voltage climb way higher than is suitable for extended battery life.
Just because the voltage is the same across all the batteries in parallel doesn't mean they will all remain at a common state of charge, one will remain virtually fully charged while one gets worked very hard because the voltage will not drop until the weakest cell is fully discharged, well beyond the 0% SOC. In severe situations the weakest cell will suffer with reverse current flow destroying the cell in a matter of minutes.
I've only had my 2 Solarking 100Ah LiFePO4 batteries, in parallel in our caravan, for just under a year, but that hasn't been my experience. So far.

Only problems I've found are:

Needed a good battery meter/monitor, & also a DC-DC/MPPT charger. I did have a VIctron MPPT, but removed it to keep the battery compartment devices down, and less cabling.

The internal BMS for each is limited to 80A continuous/160A for 10 seconds. I also thought that there was a 100A for 30 seconds in the specs., but must have read that for another brand. I can happily draw > 190A for a few minutes. Tested it with an 1800W kettle, about 1/4 full. Batteries drop from 13.6V (float) to 11.6V, then back up to ~13V when the load is removed (I.E. Water has boiled :lol: ).

Two interconnecting cables are same lengths, as are the cables to the fuse, shunt, inverter & distribution point.

parallel-battery-connection.jpg
parallel-battery-connection.jpg (17.63 KiB) Viewed 2153 times

Only diff. I have is twin cables between each of the batteries. My battery's poles are at each end. Which I much prefer.

The wires from the chargers go to the other battery +ve. All -ve cables/wires go to the shunt.

When I installed the Victron BMV-712, after our first trip, I discharged the battery bank to the low voltage cutout point of the 1st battery charger, which is also the fuses & distribution point. 206Ah. I ran the toaster, coffee pod machine & occasionally the kettle during the test. No solar or AC charging connected.

Time will tell of course. They may fail tomorrow.

I had a different meter at first, and did some testing with that, but it oveheated on one end just running the toaster which was drawing about 65A. Took it out, and it's working fine on my 48V home system. I think it was a poor joint between the mounted circuit board and the shunt. I've separated them, and run wires from the shunt to the board. So, our first trip I had no idea who much we'd had used. Ran out of juice after 3 1/2 days stopped in 1 place. No solar (or at least very little).

I did ask the seller about using the 2 batteries in parallel, and he was not concerned. It says in the specs. parallel & series is fine. They have a 5 year warranty, and there's a proper store to call/visit if I do have problems. They're an hour up the road. But like any business, they might not be there tomorrow. It happens. There's always the distributor to fall back on.

I've run the kettle a few more times recently, running the batteries down, only a bit each time ATM, so I can test the new DC-DC charger's MPPT, and my new solar panels. I may do another full capacity test before we go away. If we can go away mid June. :? <-1.5m -> :(

If I do have any probs, I'll post here.

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Re: 12v lithium batteries can't be paralleled?

Post by Adverse Effects »

you would be better dismantling the battery's and getting the cells out of them and using just 1 of the BMS's

you couldnt pay me enough to gang up a bunch of cheap ass chines BMS's
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Re: 12v lithium batteries can't be paralleled?

Post by dRdoS7 »

HI,
Adverse Effects wrote: Wed, 13 May 2020, 14:31 you would be better dismantling the battery's and getting the cells out of them and using just 1 of the BMS's
Why would I do that and void my warranty?

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Re: 12v lithium batteries can't be paralleled?

Post by Adverse Effects »

dRdoS7 wrote: Thu, 14 May 2020, 11:18 HI,
Adverse Effects wrote: Wed, 13 May 2020, 14:31 you would be better dismantling the battery's and getting the cells out of them and using just 1 of the BMS's
Why would I do that and void my warranty?

dRdoS7
so you read the first line of my post but not the 2nd?

because the 2nd line explanes what you are asking
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Re: 12v lithium batteries can't be paralleled?

Post by dRdoS7 »

Hi,
Adverse Effects wrote: Fri, 15 May 2020, 07:35
dRdoS7 wrote: Thu, 14 May 2020, 11:18 HI,
Adverse Effects wrote: Wed, 13 May 2020, 14:31 you would be better dismantling the battery's and getting the cells out of them and using just 1 of the BMS's
Why would I do that and void my warranty?

dRdoS7
so you read the first line of my post but not the 2nd?

because the 2nd line explanes what you are asking
This?
Adverse Effects wrote: Wed, 13 May 2020, 14:31 you couldnt pay me enough to gang up a bunch of cheap ass chines BMS's
I read it, and didn't think it was worth replying too.

If one BMS is OK to keep, both are.

As I said, I have a 5 year warranty, why would I void it?

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Re: 12v lithium batteries can't be paralleled?

Post by Adverse Effects »

would you put a V8 to drive 1 back wheel in a car and a 4 banger to drive the other wheel?

that will happen from the miss match in power input and output?

the "quality" (and i use the word extremely loosely) will most likely cause trouble and you wont see it till its to late

and i bet that the " 5 year warranty" isnt worth the paper it is printed on.

they will come up with some claws that will make it not applicable and not valid

you ever seen the inside of those lith battery's? i'll look for the video i am thinking of where a guy cuts 1 open
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Re: 12v lithium batteries can't be paralleled?

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If you don't have time to do it right,
When will you have time to do it over

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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

they will come up with some claws that will make it not applicable and not valid
Love it, if was intentional or not, can I use it in future or are you going to copy write it :D

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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

dRdoS7 wrote: Tue, 12 May 2020, 19:09 Hi,


I've only had my 2 Solarking 100Ah LiFePO4 batteries, in parallel in our caravan, for just under a year, but that hasn't been my experience. So far.

Only problems I've found are:

Needed a good battery meter/monitor, & also a DC-DC/MPPT charger. I did have a VIctron MPPT, but removed it to keep the battery compartment devices down, and less cabling.

The internal BMS for each is limited to 80A continuous/160A for 10 seconds. I also thought that there was a 100A for 30 seconds in the specs., but must have read that for another brand. I can happily draw > 190A for a few minutes. Tested it with an 1800W kettle, about 1/4 full. Batteries drop from 13.6V (float) to 11.6V, then back up to ~13V when the load is removed (I.E. Water has boiled :lol: ).

Two interconnecting cables are same lengths, as are the cables to the fuse, shunt, inverter & distribution point.


parallel-battery-connection.jpg


Only diff. I have is twin cables between each of the batteries. My battery's poles are at each end. Which I much prefer.

The wires from the chargers go to the other battery +ve. All -ve cables/wires go to the shunt.

When I installed the Victron BMV-712, after our first trip, I discharged the battery bank to the low voltage cutout point of the 1st battery charger, which is also the fuses & distribution point. 206Ah. I ran the toaster, coffee pod machine & occasionally the kettle during the test. No solar or AC charging connected.

Time will tell of course. They may fail tomorrow.

I had a different meter at first, and did some testing with that, but it oveheated on one end just running the toaster which was drawing about 65A. Took it out, and it's working fine on my 48V home system. I think it was a poor joint between the mounted circuit board and the shunt. I've separated them, and run wires from the shunt to the board. So, our first trip I had no idea who much we'd had used. Ran out of juice after 3 1/2 days stopped in 1 place. No solar (or at least very little).

I did ask the seller about using the 2 batteries in parallel, and he was not concerned. It says in the specs. parallel & series is fine. They have a 5 year warranty, and there's a proper store to call/visit if I do have problems. They're an hour up the road. But like any business, they might not be there tomorrow. It happens. There's always the distributor to fall back on.

I've run the kettle a few more times recently, running the batteries down, only a bit each time ATM, so I can test the new DC-DC charger's MPPT, and my new solar panels. I may do another full capacity test before we go away. If we can go away mid June. :? <-1.5m -> :(

If I do have any probs, I'll post here.

dRdoS7
The voltage drop under load says either the cells or the BMS or both are not up to a high current draw. If it really did share the load across both batteries it would only be 85 amps per battery, 85 amps from a 120Ah battery is roughly a 0.7CA draw per battery, the cells should be able to maintain 3.2v plus at that load so the voltage should not drop below 12.8v.

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Re: 12v lithium batteries can't be paralleled?

Post by dRdoS7 »

Hi,
T1 Terry wrote: Tue, 26 May 2020, 12:30 The voltage drop under load says either the cells or the BMS or both are not up to a high current draw. If it really did share the load across both batteries it would only be 85 amps per battery, 85 amps from a 120Ah battery is roughly a 0.7CA draw per battery, the cells should be able to maintain 3.2v plus at that load so the voltage should not drop below 12.8v.
What about from 100Ah batteries?

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Re: 12v lithium batteries can't be paralleled?

Post by rhills »

We have 3 x 100AH LiFePO4 batteries in parallel in our motorhome and boiling a kettle via the Inverter never pulls those below about 13V.
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

dRdoS7 wrote: Tue, 26 May 2020, 12:37 Hi,
T1 Terry wrote: Tue, 26 May 2020, 12:30 The voltage drop under load says either the cells or the BMS or both are not up to a high current draw. If it really did share the load across both batteries it would only be 85 amps per battery, 85 amps from a 120Ah battery is roughly a 0.7CA draw per battery, the cells should be able to maintain 3.2v plus at that load so the voltage should not drop below 12.8v.
What about from 100Ah batteries?

dRdoS7
This test was done back in 2011 using a 1050w coffee percolator to load test a 90Ah LFP battery pack.
4 times 1C discharge terminal voltage, 90ah LFP Winston cells July 2011.jpg
4 times 1C discharge terminal voltage, 90ah LFP Winston cells July 2011.jpg (92.45 KiB) Viewed 1963 times
Any decent LFP or LYP cell should be able to supply a 1CA discharge rate up to near fully discharged while still maintaining 3v per cell. As you can see, the discharge test was actually slightly greater than 1CA at times (more than 90 amps load) The newer chemistry Winston LYP cells suffer less voltage sag and will deliver the full 1CA load down to 100% DoD while still under load. We service customers RV battery packs roughly every 3.5yrs, although we don't see many of them until after 8 yrs continuous operation 24/7 because these are full time nomads. The load test used in this service is the same as the factory load test used when testing the cells to ensure that meet the specs, a 0.5CA load or 100% DoD over 2hrs. Even after 8 yrs of continuous service they still return the full advertised capacity without a cell dropping below 3v while under load. I guess it will be another 3 yrs or more before we get to do another capacity test and by then they will be approaching 12yrs in service.

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Re: 12v lithium batteries can't be paralleled?

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T1 Terry wrote: Mon, 25 May 2020, 12:06
they will come up with some claws that will make it not applicable and not valid
Love it, if was intentional or not, can I use it in future or are you going to copy write it :D

T1 Terry
when i first typed it it was a mistake but when i reread it back i had to laugh my self

sure go ahead i dont own it :-)
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Re: 12v lithium batteries can't be paralleled?

Post by TyTower »

T1 Terry wrote: Tue, 12 May 2020, 13:40 Lithium batteries inherently have a very low internal resistance. if you attempt to parallel multiple 12v batteries, one battery will do all the work until it fails and that will kill the other batteries in parallel. This also goes for the maximum current load and continuous current load, they will not share that load over multiple parallel batteries, so if the limit for one battery is 50 amps, no matter how many batteries you put in parallel the limit is still 50 amps. If the BMS in the battery doing all the work fails, the next in line as far as resistance gets all the load until it fails and so on. A warranty claim will be rejected because the BMS capacity was exceeded.
We do get a bit of work from making these multiple 12v batteries sort of work together after one battery in the group fails, the supplier replaces it after a fight but refuses to replace any more if they fail claiming it must be the way you installed them if they keep failing.

Recharging is the next problem, the battery with the least resistance will charge first, when it reaches the end of charge voltage the charger drops into float. To get the battery with the longest leads and highest resistance to fully charge, the battery with the lowest resistance must have its terminal voltage climb way higher than is suitable for extended battery life.
Just because the voltage is the same across all the batteries in parallel doesn't mean they will all remain at a common state of charge, one will remain virtually fully charged while one gets worked very hard because the voltage will not drop until the weakest cell is fully discharged, well beyond the 0% SOC. In severe situations the weakest cell will suffer with reverse current flow destroying the cell in a matter of minutes.

T1 Terry
Hi Terry .
Look I just don't agree with this and it seems many others disagree.
I parallel two 27 v LFP batterys to supply my inverter and separately monitor and BMS each string. As current flows out the two strings stay almost equal right down to 24 volts and charge back up equally to 27 volts.
Common sense tells you if one string goes low in voltage electrons will flow from the attached parallel string to bring it back up and even the strings out, voltage wise.

Having 8 cells in each string its a lot like the old L/A days but at least we can monitor the individual cell voltages and watch for problems.

The same happens on recharging when the voltage stays even as the charge comes up .
What were you thinking?
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

Well, I did a demo with 4 paralleled 100Ah Winston cell 4S batteries, each with a Victron 712 BMV reading the output from each battery and an inverter pulling 100 amps powering a few electrical loads. Theory says each battery should provide 25 amps, but the 100 plus people who witness the demo projected up on the big screen saw the first battery provided more than 75% of the load, the second battery started out providing around 20% and the third battery around 5%. The forth battery only provided a bit until the voltage dropped to 13.4v, then nothing, the third battery did the same and the second battery tried to provide 25 amps but was showing signs it was going to taper off and leave the first battery doing all the work. Time ran out to see just what the end result would have been, but battery one was already past 50% DOD and heading towards trying to provide nearly all the load with more than half of its capacity already used up.
I should have video'd it but I was flat out trying to do the demo in the hr we were provided and that included setting up and dismantling time as well as the talk leading up the demo explaining what we were attempting to show.
I will video the next one, if I do another that is, I don't actually have time to do my own jobs between the customer's jobs so finding the time to set up another demo 4 test packs where current and cell voltages are displayed for each battery is highly unlikely for a while. The next "Stone The Crows" at Wagga is/might be on over Easter 2021 and this is where we did the last demo so that would be the place to do the next one. A lot of RV's attend these get togethers and have to power their rigs for the 7 days from their batteries and solar, a lot that thoght they had a good system discovered they didn't ....

I'd love someone to provide 4 drop in batteries for me to do the demo with, but my reputation goes before me and no one wants to offer me any :lol: It would be a great chance to demo that what I'm saying is wrong ...... anybody ?????

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Re: 12v lithium batteries can't be paralleled?

Post by coulomb »

T1 Terry wrote: Tue, 22 Dec 2020, 13:18 ... the 100 plus people who witness the demo projected up on the big screen saw the first battery provided more than 75% of the load, the second battery started out providing around 20% and the third battery around 5%.
Well, that just shows that it's not a good idea to parallel battery modules improperly.

It's quite easy to parallel two battery modules properly, using "diagonal takeoff". Four modules or cells can also be done, it's a little more involved.

The question is whether it is possible to parallel 12 V lithium battery modules, when paralleled properly. I think that if there is a problem, it's to do with protection. If the load exceeds what the battery should be delivering, then one BMS will disconnect its cells first, putting the entire load on the second, which may damage the second module's BMS before it can disconnect its cells.

Your demo was still interesting; I thought the furthest cell would contribute more over time rather than less as the battery discharged. Similarly, I expected the first module to provide less and less of the load over time, as its internal resistance increased compared to the others.
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Re: 12v lithium batteries can't be paralleled?

Post by T1 Terry »

My theory, that I'm yet to figure out how to prove or disprove, is the cabling resistance is the cause of the poor load sharing.

Had a customer who had nightmare dramas with 2 paralleled drop in batteries from a WA supplier. The first battery had completely failed after 7 mths but was showing all the symptoms of failing from about the 2 mth of operation. Various auto electricians had replaced solar regulators, solar panels, mains chargers, cables twice including switching the main negative to one battery and the main positive to the other battery and 0 gauge cable all round ... one even replaced fuses and fuse holders .... but inside the van :?
The supplier initially refused to accept a warranty claim saying the batteries should never be paralleled, but when challenged did a side step. Apparently the agent in Brisbane had sold the customer the 2 batteries knowing full well he was going to fit them in parallel and rang the WA supplier to make sure that they would do just that, he was assured it would work and be covered by the full warranty. The Brisbane agent wouldn't accept the claim, handed it back to the WA supplier ... When the supplier was told the customer would be in WA within the week and planned to start a fair trades action, the supplier replaced the one dead battery but refused to honour a warranty on the other battery and refused to honour any more warranty as far as any of the batteries went because it now said not to parallel them on their website ......
The wife was in tears when they arrived at our workshop, the husband looked as though he was about to loose the plot completely. The new battery was doing exactly the same thing as the battery that had died and they had now spent over $10,000 and no closer to the dream of being able to free camp.

I made an equal length cable set up with each negative and each positive going to a single common point and the main cables running off from there. The batteries sort of shared light loads, one would supply first, then the other would take over and the first battery would stop supplying .... then after a while switch back. So not 50% load each, but loaded 50% of the time each ... sort of ..... At the 100 amp load there was load sharing, but 80% - 20% and switching back and forward charging was a bit the same .... but they were much happier and we received an email mths later to say they could finally use their van for free camping, but still watched the battery voltage and load from each battery and were not game to go below 50% SOC ..... so still not really getting the full benefits of a lithium battery, but at least better than they had before.
Lots of tales of woe appearing on the many RV sites now with people who had fitted 4 drop in batteries but were not getting anything like the capacity expected, but that could be the capacity being dogie and not what the label said it was.

T1 Terry
Green but want to learn
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