Power Consumption for Dummies

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Post by scooterage » Fri, 27 Feb 2009, 06:16

so how do i, charge it seperately with what i have? do i just creat a connecter between the charger port i have and put seperate wires to that cells connection like u did today?...if so how do i know on the battery which is positive side and which negative? i wonder what the cut off voltage is for this pack?..cause today that problem cell was at 1.2 when it was cutt off, but tonight it was at 2.8v...

and antiscab said the ping batteries are 3c?...is that when the bms is disconnected when discharging?...cause the ping description said 1C rating.... if not can we make it 3c, will that help it any better, even if the one cell was fine?
i was thinking once this battery pack is hopefully balanced or replaced, maybe we could (if possible,with how you described today) add the other lithium battery cells to the pack to increase the continuous max and peak max on the board... would i just parallel the batteries up pos to pos---neg to neg?...would that be the easiest way to add 10ahs? or do would i need to do it individually with the cells?....if we did that i would probably need a higher charger than the 2.5 charger i have, is that right?
Last edited by scooterage on Thu, 26 Feb 2009, 19:21, edited 1 time in total.

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Post by antiscab » Fri, 27 Feb 2009, 06:39

to charge separately, you need another powersupply.

the pack is probably limited to 1C for thermal reasons. 3C should be fine for short durations.

you could parrallel the two packs. as long as both packs have the same number of cells in series, it would work fine.
I would try parralleling the positive and negative leads that come from the BMS first, rather than bypassing the BMS if you intend to parrallel the pack permanently.

the charger should be fine charging both packs in parrallel, it will just take longer to achieve full charge.

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Post by Electrocycle » Fri, 27 Feb 2009, 14:13

yeah charging the cell on its own will require a separate power supply, preferably one with the voltage limited to 3.8v or so, and would require careful attention.
The way I charged it up is not safe to do without supervision, as the charger wasn't limited in voltage.

You could actually parallel both of your complete battery packs for more range - but the dodgy cell would need to be replaced first.
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Post by Johny » Fri, 27 Feb 2009, 14:45

It may be enough to leave the pack on the normal charger for a few days. You mentioned that ping said this was OK and it will give the BMS time to balance the pack.

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Post by scooterage » Fri, 27 Feb 2009, 15:55

i have a car charger i think is 6v-12v charger...will that be ok for to charge the one cell if i monitor it with the voltmeter at the same time making sure the cell doesnt go over 3.8v?

just to note ..when it cut out and i took it home and check the voltage it read 2.8 and the highest read 3.4...so it is still behind .6-.7v from charge (3.8--and bad cell 3.2)to discharge (3.4--and bad cell 2.8) so it hasnt discharged any faster than the other cells, it just hasnt caught up to them on the full charge...is that right? but when charging the 3.2 wont go any higher than 3.2

i also read on the motor and found specs on its details if this helps
Model:MY8922
Voltage/V             36V
Rated Power/W        800W
No-load Current/A     ≤3.0
No-load Speed/RPM     3700
Rated Torque/N·m      2.55
Rated Speed/RPM       3000
Rated Current/A      ≤27.8
(on the actual motor it says rate current 28.5amps)
Eff./%                 ≥78

with these details does that mean the motor at its peak use say for going up hill may require 27.8a X 3.0(no-load currant/a) = 83.4amps
or is it just double as ping thinks it may require at peak 55.6...and because its continuous peak is higher than the batteries Discharging Cut-off Protection: 40-50 Amps (tho batteries peak is 60amps) this could also be the reason for it also to cut out aswell as the bad cell?
and would the extra 10ah lithium(sorry i put 12ah lith before) added in parrallel increase not only the overall AH but the overal max continuous amps and peak of the batteries when put together? or does the BMS on each limit both batteries individually to its original specifications?



even if the cell is damaged should the all the cells be losing .4 volts within 15mins that would mean if the cells where actually even and the actual cut off voltage was say 2.5 volts on the cell, wouldnt that mean the battery only will last for about 45mins?...which is the same length of time my sla battery lasted but this thing has more amps and i could have bought a fair few more sla's for that price of this lithium and gotton further. aaaahhh!!!

this morning i just checked all the cells before i charged it...they all read about 3.4v.....but that one had increased to 3.2 since last nights run which left it 2.8.....then i charged it for a short timeall the other cells increase at the same levels and get to about 3.8...but the problem one just stayed at 3.2....if i leave this on charge again today for the whole day even, i dont think that cell will go past 3.2
Last edited by scooterage on Fri, 27 Feb 2009, 08:19, edited 1 time in total.

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Post by Electrocycle » Fri, 27 Feb 2009, 20:02

hmm.. It'd be worth leaving it on for a while to see if that cell does come up (the problem is that because the charger is cycling on and off due to the other cells being full, the bad cell only gets a very slow charge.
If you connect a 6v car charger to the bad cell and monitor it with a volt meter till it gets up to the same voltage as the other cells you should be ok.
I'd try to only use a charger with a low current mode (2A or so) if possible, and be very careful about letting the voltage get too high.

If you put the other 10Ah lithium pack in parallel with the 20Ah one you'll end up with a 30Ah pack with the discharge current capability of both packs added together.
You should be able to parallel the charging connections as well, and use the same charger, otherwise you can use two chargers.


The motor's peak current would be hard to guess, but it's not too hard to measure with a current shunt and multimeter.
I would have measured it yesterday but we didn't have the board working.

No load current just means the amount of current it draws while spinning freely in the air, and the rated current will be the current it draws while putting out 800w of power.
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Post by antiscab » Fri, 27 Feb 2009, 21:43

ideally all cells should be above 3.4v just off the charger.
any that arent, arent fully charged.

just using the ping charger and bms, it will take days on charge for that low cell to catch up.

the amount of energy between 3.8v and 3.3v at no load is *very* small.
the difference between 3.3v and 3v at no load represents 80% of the rated capacity.

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Post by scooterage » Wed, 04 Mar 2009, 15:33

[quote="Electrocycle"]
If you put the other 10Ah lithium pack in parallel with the 20Ah one you'll end up with a 30Ah pack with the discharge current capability of both packs added together.
You should be able to parallel the charging connections as well, and use the same charger, otherwise you can use two chargers.


with paralleling the charging connections, will either battery feed off the other, whether while the motor runs or the power to the board is turned off? basically if the batteries are just sitting there will they feed off each other and with the any excess power just wasted down through either or both bms's?

Last edited by scooterage on Wed, 04 Mar 2009, 04:35, edited 1 time in total.

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Post by Johny » Wed, 04 Mar 2009, 15:55

Providing both packs have the same voltage when charged, they will be safe to leave in parallel. Charge one pack, leave it sit for an hour and measure the voltage. Charge the other pack, leave sit for an hour and measure voltage. If voltages are the same +/- about 1 volt then they will equalise to each other and can be left paralleled all the time.

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Post by scooterage » Thu, 05 Mar 2009, 02:56

ok, but electrocycle said i could put the charging wires together to charge them both through one charger at the same time, it may take long to complete the charge but is possible. my consern is while not on charge, being they would be both connected to each other pos to pos neg to neg, they may try to feed off eachother for charge through the charging wires?
if this happens i was going to only keep the discharge wire connected, and charge seperately.
the next question then would be would it be safe to charge both packs at the same time with seperate chargers if the discharging wires are still connected but the charging wires obviously in this case would not be connected. the 10ah charger is 1.8amps the 20ah charger is 2.5amps...
or would it be better just to charger both batts completely unwired from eachother?

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Post by Electrocycle » Thu, 05 Mar 2009, 04:55

I think it would be fine to parallel the charge wires and charge from the same charger.
It'd be worth checking that both packs finish charging at the same voltage, but each pack's BMS should keep it under control.

Since both packs will be at close to the same voltage they won't feed current back and forth between them.
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Post by antiscab » Thu, 05 Mar 2009, 05:09

no, if you are going to wire them together, do it permanently.
electrically, the charge and discharge wires are both hooked up the same way.
so if you disconnect just the charge wires, the batteries will still be connected together via the discharge wires.

the batteries wont try to discharge each other, as long as they are both fully charged when initially connected together.

it is safe to use both chargers at the same time, as long as you do it through the charge wires.
the charge time will be shorter this way.

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Post by Johny » Thu, 05 Mar 2009, 14:37

Agreed, connect them permanently. For peace of mind I would still do the independent charge, rest, and measure thing just once so you know the the final pack voltages are pretty much the same.

Then when you are satisfied, connect them together forever.

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Post by scooterage » Thu, 05 Mar 2009, 15:57

the main thing i was concerned with charging both at the same time with the dishcrge connected permenantly or otherwise (the positive on both being a charge/discharge wire) was...
A: i wasnt going to kill myself or the batteries having two chargers going into two batteries that are directly connected to each other via the positive side charge/discharge wires.
B: being that the 10ah batt uses a lower amp (1.8) charger the (2.5) amp on the 20ah batt hopefully wont affect 10ah on charge since only the pos wire is directly connected to eachbattery on charging and not the neg charge wire.
C: if both batts are completely connected together wires and using the 2.5 charger on both at the same time, when one batt is full how does the other get charge? (as the 10ah bms seems to have a much better ballancing system, it would charge the battery showing red on the charger and would turn green when finished and completely balance, that was it, where as the 20ah bms would have the charger on red then green when one cell hits 3.9 then turn red again 5mins later.

well i guess i will have to just test it. i just wanted to make sure i wasnt going to kill the batteries, myself, or burn the house down which ever way i connect them, fully together, or half seperate on the charge side LOL!!!

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Post by Electrocycle » Thu, 05 Mar 2009, 16:35

the charger won't turn off until both packs are fully charged.

Each pack's BMS will balance its own cells and cut off the charge when it's full. The packs will charge at the same rate because the charger voltage will be held down by whichever pack is drawing the most current.

One charger into each pack would be fine, but for convenience I'd just use the one charger doing both. It'll be a bit slower, but if that's not a problem it's simpler at least ;)
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Post by scooterage » Thu, 05 Mar 2009, 22:14

No worries, thanks heaps everyone for all your help so far, ping has a cell and a free dc power supply for the cell on its way in the mail, once ive put it to the pack and chargered it i'll let you know how it all goes, same with putting the two packs togather,
Thanks again!!!

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Post by gpsnettrack » Fri, 13 Mar 2009, 07:34

I am glad this is the dummies thread Image

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Post by Richo » Fri, 13 Mar 2009, 08:06

hijacked Image

Point: Start your own thread Image
So the short answer is NO but the long answer is YES.
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Post by kiwi_nigel » Thu, 07 May 2009, 20:54

Hi,

I run these motors on electric skateboards with a 36V 16Ah LiFePO4 battery pack and they're fine.

But, have tried in the past with a 10Ah pack which was too small - pack wasn't necessarily, the BMS was - it kept cutting out!

Note: the motors draw 46-50Amps when heavily loaded which is right at the top end of the smaller BMS range.

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Post by Magneto » Sun, 31 May 2009, 14:21

Could I please get some advice regarding a battery problem.

My Ev has 12 x 12 Volt lead acid deep cycle batteries. I have been advised that for the long term life of the batteries I should not allow then to discharge under 10.5 volts each. That is a total of 126 volts for the entire bank of batteries.

I notice on my metre that when the batteries are fully charged the metres shows about 152 volts when I start driving (foot off the accelerator and the batteries not under any load). That is after the surface charge had been quickly wiped off. When I accelerate and put the batteries under load the metre would probably show a voltage of 144 volts. I will drive along for a while with the voltage dropping, but indicating a higher voltage whenever take my foot off the accelerator.

Thus whenever I need to increase the load on the batteries by accelerating, especially travelling up hils the metre will indicate a reduced voltage, returning to a higher voltage, whenever I take my foot off the accelerator.

My question is that should the metre begin to display a voltage less than 126 volts (ie less than 10.5 volts per battery) when the batteries are under load, should I consider the batteries fully discharged and stop the EV or should I only consider the batteries fully discharged when the voltage drops below 126 volts , when the batteries are not under load (ie I have taken my foot off the accelertator and allowed the voltage display to return to the higher voltage when the batteris are at rest?

Advice would be appreciated.

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Post by antiscab » Sun, 31 May 2009, 22:27

do you have a link to the data sheet for your batteries?
what is they're brand and model number.

in general, the 10.5v limit is under load.
reason is, at 10.5v, it is very likely 5 of the cells are at 2v, and the 6th at 0.5v.
below 10.5v, and you risk reversing one of the cells (and greatly reducing its service life).

If you had a way of *knowing* that all the cells still have charge, then you could let the voltage go lower safely (as you would know its just sag, and not a cell or two reversing).

measuring voltage at the pack level isn't a great way of working out end of charge.
the 10.5v number is for measuring at the 6cell block level.
a 10.5v average could mean a few cells are being reversed.

you could buy a paktrakr, so you can see the voltage of each 6-cell block *all* the time. Then its just a matter of backing off the accelerator when any one battery goes below 10.5v.

a cheaper solution would be to have a 12v meter, and measure the voltage of just one battery. That would require you finding the weakest battery (and checking from time to time to make sure it is still the weakest).

another thing i would do is set the battery voltage limit in the controller to 126v (the controller can react alot faster than you can to back off the accelerator, and it will back off only by what is required).

anyway, in summary, the 10.5v is at load.

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Post by Magneto » Mon, 01 Jun 2009, 02:37

Matt

My batteries are Optima Deep Cycle 55 amp.

After charging all batteries but one are showing 13.5 volts. The weak one is showing 11.4 volts, but quickly decreasing to 10.9 volts.

It looks like I have a reverse cell. Is there anything I can do about it?

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Post by coulomb » Mon, 01 Jun 2009, 05:59

Magneto wrote:It looks like I have a reverse cell. Is there anything I can do about it?

You can try overcharging it carefully. It'll probably need replacing, so you may as well try, assuming you can't get it replaced under warranty. What I'd do is take that battery out of the pack, find the absolute max voltage you are allowed to charge them to, temperature compensate it, and apply a fixed voltage (adjusting for temperature as often as is convenient) until the rest voltage (after at least an hour rest) comes up to normal. It might need this for days.

Because these are valve regulated, it's important not to let them gas so much that they have to vent, since the few grams of water will escape. Hence the need to get the voltage just right: too high and it will vent and be ruined; too low and it will never charge the low cell. It might be worth attempting to start this charge when it's been discharged to say 10 V with a suitable load (headlights, etc).

If you are more adventurous, you could attempt to open it to get at the individual cell terminals. (This will void the warranty, but I'm assuming you can't get this fixed under warranty anyway). Then you can find the cell that is really low, and charge that one at up to 2.4 V and up to say 10 A until it seems to come good, then give it a good charge. That way, you don't heat up and risk venting the good cells.
Discharge the whole battery to about 10 V, checking the bad cell for weirdness. If all is well, give it a longish normal charge, and try it in the pack again.

While all this is happening, your EV may (or may not) be driveable, with less range and possibly a lower high speed limit. Be careful not to stress the others if you do this (by forgetting that you have less range temporarily). I'm assuming that you don't have a spare Optima to replace the bad one with.

As an absolute last resort, you could consider attempting to apply a few drops of distilled water to the bad cell, in case it vented in the past. It may come back with water added.

You'll want a good power supply or charger where you can accurately set the voltage and current limits for parts of this. Google extensively for first hand accounts of what people have done and the details of how (e.g. exactly how to get the water in there safely, how to open the battery, etc.)
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Post by Magneto » Mon, 01 Jun 2009, 17:24

Coulomb

Thanks for the advice. I don't think that I will be that adventurous as to try and get into the interior of the battery.

I think that my headlights might operate off a small 12 volt battery which is on continuous charge from the main bank of batteries.

That is the main battery pack sends charge through a 12 volt DC convertor to the small battery to operate all the 12 volt equipment on the car.

So if I try to supply a load through leaving the headlights on whilst the main bank of batteries is charging then, it may not be putting the main bank of batteries under load. (I will have to check with the guy who built the EV)

Can a battery with as reversed cell affect other batteries in the main bank of batteries if I keep running the EV?

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Post by acmotor » Mon, 01 Jun 2009, 18:07

Magneto, Best advice re the optima :-

EV batteries 101 Image

If you have found the faulty battery, replace it now.

Even if you can """"recover"""" some function you are kidding yourself as it will NEVER be reliable and WILL have a very shortened life. It will also risk destructive failure. Image

The old chain theory is correct. Remove the weakest link.
You are fortunate in being observant enought to have noticed there is a problem and track it down.

Batteries (lead acid in particular due to their weight) suffer from shipping and handling damage that can case shorted plates, loss of electrolyte, damaged terminals. There is the occasional manufacturing fault (batch date 4:37pm Fridays) but mostly physical damage.
You can kill a battery by misuse but it sounds like you are not into that ! But then the optimas will give only a few hundred deep cycles anyway. Image

I say again. Replace now. Move on. There is no majic elixa !     Image   
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