Replacing a dead cell

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Taxman
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Replacing a dead cell

Post by Taxman » Sat, 01 Jun 2013, 15:47

Hi guys,

So I bought the Datsun ute that was advertised on this forum a while ago, trucked it down to Brissie, pretty stoked with the thing. It's got a 144v 75Ah lead acid pack, and one of the cells is pretty much dead.

Apparently the pack has only got around 100 cycles on it, but I get the impression it has been pushed pretty hard (It came from Townsville, where I think hooning is a pretty popular pastime).

I was planning to upgrade to a 160Ah lithium pack, but I've blown all my cash, and was hoping the lead acid pack would see me through til I can get the money together.

My question is - is it worth replacing the one dead cell? Is another cell just going to die straight away anyway? All the others are holding good voltage, testing within .05 volts of each other, after a run around the block.

Also, does anyone know where I could find an optima yellow top in Brissie? Best price I could find is 360 bucks coming from Victoria.

Any thoughts would be great,

Sam

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Jeff Owen
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Replacing a dead cell

Post by Jeff Owen » Sat, 01 Jun 2013, 16:35

Taxman
Is there some form of battery management on the car?
What makes you think one of the batteries is dead?
Have you tried charging the offending battery on it's own?
Where are you in Brisbane?

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Replacing a dead cell

Post by Taxman » Sat, 01 Jun 2013, 18:01

Hey Jeff,

There is no BMS, I just tested each cell voltage after taking it round the block, they are all around 12.95v, the bad guy is at 11.5. I assumed there is nothing I could do - do you reckon it's worth just sticking that one on charge with a 12v charger for a while?

Cheers,
Sam

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Replacing a dead cell

Post by Taxman » Sat, 01 Jun 2013, 18:02

I'm in West End also, by the way.

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Jeff Owen
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Replacing a dead cell

Post by Jeff Owen » Sat, 01 Jun 2013, 18:35

The lack of BMS is probably the cause of the problem. The pack is out of balance. Charging the pack as is will likely damage more batteries.

I think you need to charge the low battery but you will need a charger that goes to 14.7V (From memory 17 years ago). I have one for Hawker batteries which are similar. I am running late for an appointment but will get back to you later today

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Replacing a dead cell

Post by BigMouse » Sat, 01 Jun 2013, 18:36

Taxman wrote: Hey Jeff,

There is no BMS, I just tested each cell voltage after taking it round the block, they are all around 12.95v, the bad guy is at 11.5. I assumed there is nothing I could do - do you reckon it's worth just sticking that one on charge with a 12v charger for a while?

Cheers,
Sam


It may just be out of balance. Definitely worth giving it a full charge on its own. 11.5v immediately after reasonable current being drawn isn't -that- bad. If you already have a 12v charger, it won't cost you anything to try!

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Simon
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Replacing a dead cell

Post by Simon » Sat, 01 Jun 2013, 20:35

Definitely worth replacing that battery. 11.5V after a run around the block is pretty bad. What sort of charger does the ute use? If you can adjust the Voltage you could just bypass the bad battery.
Don't think you will find any cheaper price than that. My 55AH Optima's cost $330 each.

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Replacing a dead cell

Post by Taxman » Sun, 02 Jun 2013, 21:10

Okay, so I charged up the suspect cell seperately with an ordinary 12v charger. It had 13v coming off the charge, an hour later it had dropped to 12.3, after a few hundred metres of driving, it's back to 11.5. So I think that is the death knell for that one.

So is it worth replacing the cell? Am I going to end up replace the whole pack, one at a time as they die?

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acmotor
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Replacing a dead cell

Post by acmotor » Sun, 02 Jun 2013, 22:30

Sounds like that 12V BATTERY (6 cells) has one shorted CELL and the 13V you see coming off charge is the other cells in the 2.6V+ gassing region that they won't stay in for long. A short loading and the battery will drop to 11 or so volts.
Repeated charging will only kill the rest of the battery pack if charger voltage is not reduced.

Yep, without access to that one cell, replace the battery.

Without a BMS at at least the 12V battery level, unbalance and a resulting dead cell occurs while the rest of the cells can still have 60% capacity. So replacing one battery and giving the others an individual charge every now and then will give you some more life.

There is a downside of course.
Installing one new battery if its Ah capacity is significantly higher than the rest of the batteries (and without a BMS to monitor things) will risk overdischarging other cells in the pack unless you are careful.
The bottom balancing technique may be required. (though I am not a fan)
I would never take the SOC that low anyway.

It may be possible cut into battery case and short the one offending cell and adjusting (by a few volts) the charger end voltage. But as cells die the system voltage will lower and current for same kW at motor will increase.

Another option, requiring some attention is that the optimas can be replaced (physical space to be considered) with Lithiums one optima to 4 lithiums as your budget allows and optima failures occur.
I would be setting up a BMS and paying attention to charging though. At least a BMS on the lithiums.

160Ah Lithiums may be an overkill and occupy twice the space of the optimas ? Even 90Ah Lithiums will give you more than twice the achievable capacity you (should) have now. I guess you've given some thought to that.

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Replacing a dead cell

Post by T2 » Wed, 05 Jun 2013, 06:34

My own experience is this. Unless you determine the longevity of the remaining battery this could be the start of "death by a thousand cuts".

My advice is that you bypass temporarily the bad cell. This will allow you to drive the vehicle with at least 90% of its performance in order to determine whether there are likely to be any further candidates for imminent replacement.

On that basis you may decide to bite the bullet and order up a new pack. In which case it can save you a great deal of grief if you negotiate the price for arranging to have the pack drop-shipped at your house and at the same time have them remove the "empties". They may offer to haul away the unwanted Lead-acid anyway free of charge. Certainly do not underestimate the time and effort of physically lugging 12 batteries back to the store ! Nor the frustration of having to swap out individual batterys every few months if you don't do this.

The problem is that Pb-acid are not manufactured at the quality they used to be. It has been speculated that since the cranking time for today's vehicles equipped with electronic ignition is probably a lot shorter, lower performance cells can be tolerated. Of course I'm assuming that these are not of the 6-volt deep discharge golf cart variety.

From what I have read here at AEVA the general concensus is to avoid Pb-acid entirely - don't throw good money after bad - and invest in a lithium pack along with a BMS about which there has been some good advice on this thread already.
T2

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acmotor
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Replacing a dead cell

Post by acmotor » Wed, 05 Jun 2013, 07:00

Good advice T2.
Replacing a whole lead acid pack in the year 2013 does seem rather a backward step.
Nurse the old pack along while you look to the future.
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Replacing a dead cell

Post by Taxman » Mon, 10 Jun 2013, 15:09

Thanks guys,

So I replaced the dead cell, but I don't plan on doing that again - so if another one dies, I'll bypass it. I won't spend any more money on this pack. I was just keen to get the thing on the road, having just bought it.

I have a complete nooby question - I'm top balancing the pack now by with a little 12v charger - I can charge them seperately with the whole pack still strung together, right? Will that affect the accuracy of the charger at all? I have taken off the cell connects just in case.

Cheers

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Replacing a dead cell

Post by BigMouse » Mon, 10 Jun 2013, 16:38

Taxman wrote:I have a complete nooby question - I'm top balancing the pack now by with a little 12v charger - I can charge them seperately with the whole pack still strung together, right? Will that affect the accuracy of the charger at all? I have taken off the cell connects just in case.

Cheers
As long as the battery string is open (contactors open), charging an individual battery shouldn't be a problem. Just make sure that you don't try to charge the whole string at the same time you have a charger connected to an individual cell. If there is any lack (or poor) isolation in either chargers, you can end up with a nasty short circuit. Only do one thing at a time.

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acmotor
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Replacing a dead cell

Post by acmotor » Tue, 11 Jun 2013, 03:02

It is worth noting that more than one EV lead acid battery equalisation system is based on individual 12V chargers ! Not as good as cell based BMS but better than nothing.
The 12 x 12V chargers ensure better top end balance but don't help with the bottom end discharge. You can always find the weakest battery, then monitor that with even just a multimeter to help preserve the pack.

Red suzi used 12 x 48V chargers with basic zener eq across each 12V battery on a 576V pack.
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Replacing a dead cell

Post by BigMouse » Tue, 11 Jun 2013, 04:00

acmotor wrote: It is worth noting that more than one EV lead acid battery equalisation system is based on individual 12V chargers ! Not as good as cell based BMS but better than nothing.
The 12 x 12V chargers ensure better top end balance but don't help with the bottom end discharge. You can always find the weakest battery, then monitor that with even just a multimeter to help preserve the pack.

Red suzi used 12 x 48V chargers with basic zener eq across each 12V battery on a 576V pack.


Yeah, that's not a bad method in place of BMS on lead acid packs. The chargers have to be specifically chosen to be fully isolated to prevent short circuits through the ground terminals. If throwing on a random charger that you have laying around, whose isolation is unknown, it's safest to make sure it's the only thing plugged in.

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Replacing a dead cell

Post by acmotor » Tue, 11 Jun 2013, 04:20

Right on about the insulation.

I tested the chargers I used with a 1kV megger from mains and earth to output DC and they all measured >1000Mohm, as they should. But that will not always be the case. Though if there is a connection to ground it will have been deliberate inside the charger and could be removed.
Note... the EV chassis should be definitely connected to mains earth and an ELCB/RCD be on the mains supply to comply with electrical safety standards.
Taxman, note that also with your single charger otherwise a fault could make the EV chassis live and undetected.

I also had 3A fuses (both wires) on the 48V side just in case.

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