sealed lead acid batteries in series

[Bill Wright]

After less than a year of use and less than 100 charge/discharge cycles, the batteries in 'Baby Bug II' seemed to be knackered. 'Baby Bug II' is Hil's smallest disability scooter. The batteries are 12V 17Ah deep discharge types, medium price. They are used in series.

Having had battles with suppliers in the past over faulty batteries I decided to get some data before complaining. I left the Bug on charge overnight and for a morning then removed the two batteries and discharge tested them. One seemed to be more-or-less all right but the other had a low starting voltage and discharged down to 9V in hardly any time at all. I fully discharged both batteries overnight, then charged then individually, starting at 2A, for 24 hours. When I did another discharge test they both seemed OK. I fully discharged and recharged them. I didn't believe they would stay OK but I put them back in the machine and returned it to service. That was three months ago and so far the batteries have been fine.

Well, now then! What do you make of that?

A single battery used by itself is actually six cells in series. On a general, battery-philosophy point, it occurred to me that when two batteries are used in series there are twelve cells in series. In what way is this different in terms of inequality of capacity and performance? It's all very well saying, "Ah, the charge to each battery has gradually got out of balance," but how is that different in principle to the cells in one battery getting out of balance? Does that happen? If so is there any mileage in fully discharging and then slowly recharging an apparently knackered battery?

Bill

[harry]

It could be the act of removal fixed the problem somehow. With non "gell cell" batteries debris from the plates used short plates out. A bit of vibration caused the debris to fall to the bottom of the cell.

[T i m]

I assume this was on a / the 'automatic' charger that cut off (or went into trickle / maintenance mode) once it determined the batteries were charged?

Not good (9V discharge level as it represents well below 50% DOD).

How? And when you say 'fully' ... ?

Ok, the 2A sounds good for a sealed / gel battery.

Ok.

Down to what voltage on what load (and current)?

Ok.

That I further believe charging multiple batteries in series using an automatic charger isn't a good idea. Also, some chargers that are available for such jobs aren't ideal.

Yup.

It isn't, but it can be part of how a charger fails to deal with that combination effectively.

Because it (the error) is more likely to be 6x <cell voltage> rather than 1x <cell voltage> and as the charger is looking for voltage ...

Yes. different 'batteries' may be treated differently but would tend to be treated the same re each cell within that battery (eg, charge current / voltage, temperature, cycles, construction). Are the lighting or controller 12V for instance?

Discharging (I don't like the undefined use of 'fully' here) and

*properly* recharging a poorly maintained battery can help it reach it's full potential, yes. It could also be the case that a sulphated battery could be recovered do some degree by the use of a desulphating charger, over it's everyday one.

My EV uses 8 x 6V 200Ah Crompton LA *wet* traction monoblocks (batteries).

With the (electro-mechanical) controller in 'off', all the batteries / cells are in series (so ~48V). However, the onboard charger is a linear one and so will automatically taper and float charge all the cells equally, with any cells receiving a surplus of charge able to vent that off.

They are also used in series / parallel combinations (12, 24 and 48V) and so the 'use' is spread across all 8 batteries fairly evenly.

Dad > Mum now has a 24V scooter that also uses the 17-20A 12V gel batteries and the supplied 24V automatic charger fails to correctly switch into trickle mode. If she ever needs to use it herself I intend to re-wire the charging circuitry to take 2 x Optimate2 intelligent

12V chargers.

The other day a neighbour asked if he should replace the (expensive) batteries on his wife's scooter. I offered to check them out for him. I first charged them individually on my Optimate6, then discharged them on my LVD test rig (down to 10.5V or higher), recharged and then discharged again, monitoring the discharge voltage and time and plotting the discharge graph for both. As it turned out, one battery had a higher capacity than the other but both were not bad in general (certainly not worth replacing for her typical usage).

Considering the cost of these batteries, charging them individually using quality chargers makes good sense (to me anyway). ;-)

Cheers, T i m

[Andy Burns]

Have you checked the charging voltage and current?

For deep cycle it should typically be 29V at no more than 4.25A, how warm is the area it 'lives' in during charging? (voltage ought to be

48mV lower per °C above 20)

[Harry Bloomfield]

Bill Wright laid this down on his screen :

IMHO batteries and cells in series do get out of sync with varying capacities, they will never be a precise match, hence the need for them to be individually charged, or occasionally given a balancing charge to bring them all up to full.

Unfortunately, none of the chargers offers a 'balancing charge facility', they charge to a voltage, then shut down. So your only option is to charge individually with a charger which will force some balancing current through them, so all the cells in a multi-cell are brought up to a full charge.

[Graham.]

But on an SLA you haven't got access to the individual cells, your only option is to charge the two batteries separately. Statistically the imbalance is more likely to occur within a given battery.

Perhaps we need more sophisticated SLA batteries with built in monitoring.

[The Natural Philosopher]

you dont becaus in general unless the battery is seriusly f***ed, the end point voltage isn't really critical.

[newshound]

+1.

Also if you trickle charge for a while that will eventually get every cell up to its full capacity. Individual cells will have a slightly different capacity and a bad one may have virtually none. A good sign of that is a battery which apparently charges OK, but drops to ~ 10 volts (i.e. 5 cells) after not much discharge. I've just taken a battery like this off my horsebox. It will probably be fine for a while running electric fencers though.

[Dave Plowman (News)]

Thing with batteries is they tend to vary enormously in quality. I'd say it makes sense to stick to the well known brands like Yuasa. Since a decent life will likely pay overall.

[Bill Wright]

The charging was. The discharge was via huge wire-wound rheostats.

The discharge was via huge wire-wound rheostats. The batteries were down to a fraction of a volt.

The batteries were down to a fraction of a volt.

But if it makes sense to charge the two batteries individually, wouldn't it make even more sense for every cell to be charged individually? I think that's what I'm getting at really.

Bill

[Bill Wright]

As it happens we have four chargers. Three are 2A max and one is 4A max. They all start at the rated max and taper off, then they all switch to a mA level at about 28V. They vary a bit in that respect.

I made up a harness that goes between the charger and the machine. It lets me monitor voltage and current. I did this when we had problems with 'Medium Bug'; battery problems again. That time it was very cheap batteries supplied by the importer. Both were f***ed after not much use. I got them a lot better by discharge and recharge, but not really good enough. I think I told this tale at the time. I think these 17Ah batteries were down to something like 3Ah. After treatment they were about 8Ah. Not good enough.

The room temperature is a constant 21.5C.

Bill

[Bill Wright]

Yes they definitely do.

Bill

[Theo]

I think the thing with lead-acid is that they will accept overcharge, which they will happily release as heat without ill effect. So, as long as the current isn't too high, you can leave them on float charge until all of the cells are fully charged. The cells that charge first won't mind being overcharged. In that instance there's no difference between charging 12V separately or 24V in series, assuming chargers are otherwise equal.

With other battery chemistries, continuing to put current into fully-charged cells will damage them - which is why the charger/BMS will balance so they all reach the top (or the bottom) at the same time. For that it needs access to the individual cell voltages.

Theo

[The Natural Philosopher]

I wouldn't say it does make sense to charge them individually.

[T i m]

Ok.

Ok (in principle).

Ouch. ;-(

Two things there, depending how long you leave the batteries in a state of no charge, (soft initially) sulphation can take place and effectively reduce the surface area of the plate's contact with the electrolyte (and hence capacity). The second being that if you take the battery down that low, because there *will* be difference in capacity between the cells, there will be one cell that discharges first and then it is likely to suffer being reverse charged by the voltage remaining across the other cells.

The result of both of those is the likelihood of a reduced cycle count and / or a damaged cell / cells.

As I said ... ouch.

Now as an experiment on some potentially 'duff' cells I guess that could be considered worth a try ... but not on a battery you believe may be good but has not been charged properly.

It would be and often is in some situations / chemistries.

Yes, I understand.

In the 'old days', batteries (the term for a series of cells) often had external terminals between the cells, or were made up from individual cells (milk floats, submarines, UPS's) and therefore you could monitor every cell in the battery.

Vented wet cells can tolerate overcharge (better than sealed / recombination) where the weaker cells, already at 100% charge could carry on being overcharged safely whilst the higher capacity cells 'caught up'. Gassing is in fact fairly important to the even charge up the height of a plate as the movement of the electrolyte prevents stratification (of the strength of the electrolyte > charge).

With gel, excessive gassing can cause issues with the gel electrolyte and it's contact with the plate, hence why generally they are charged with 'intelligent chargers that drop the (often constant) current once they have completed the bulk charge phase (and before gassing would typically become an issue)

On my discharge rig I use an adjustable 'Low voltage disconnect' that can be programmed for (typically) as low as 10 and as much as 11V before it disconnects the discharge load.

Cheers, T i m

[Terry Casey]

Bill, if you get a recurrence of the problem (and, as nothing else has changed, it seems more than likely) - would it be easy to fit a different connector allowing access to the centre point of the two batteries, which would allow you to use two chargers, each charging one battery, simultaneously?

It won't do anything to explain the problem but, as you seem to have found a solution, it could avoid the inconvenience when the buggy unexpectedly 'fails to proceed'.

[Dave Liquorice]

The one that appeared knackered started with less capacity than the other. So each discharge would take into a lower state than the other battery. If this discharge point is too low the battery won't fully charge so the next discharge takes it a bit further past the "too low" point. Repeat and eventually it'll have so little capacity the connected kit gives up. With a scooter the current draws are going to be quite high soa less than ideal battery will cause the scooter to stop before serious damage is done to the battery. Doing the couple for individual discharge charge cycles has recovered the "knackered" battery suffciently well so as to not to "die" significantly before the other.

[T i m]

<snip>

<snip>

Just OOI, my mates, wife's scooter is a fairly large one (bigger than a Shoprider) and I think we measured about 8A on full speed with the driving wheels off the road and about 10A with him on it going along the (level) road. ;-)

Discharging the batteries at about 8A down to 11V took about 100 minutes.

Cheers, T i m

[Johnny B Good]

You can't subject SLAs to that sort of abuse. A balancing charge relies upon being able to allow the cells to gas, along with a means of adding distilled water to make up the loss of the 2H2 and the O2 molecules.

Aside from the absence of a topping up facility, the gas bubbles will become trapped in the Gel electrolyte of the Gel type or the absorbent glass matt used to immobilise the electrolyte in the AGM type of SLA.

It's the reason why the SLA battery manufacturers recommend an optimum charging voltage of 13.8v for their 6 cell batteries (13.5v to 13.6v is a less destructive recommendation for continuous float charging conditions

- eg UPS battery packs).

One has to assume that SLA battery manufacturers go to great lengths to make sure the cells in their batteries are as closely matched as the manufacturing technology will allow[1] (far more so than in the case of flooded cell low maintenance SLI batteries) in order to mitigate this absence of a charge balancing feature.

That cell matching won't necessarily extend across two seperate 12v SLAs even if from the same production batch so it would help if each 12v battery can be charged to the recommended 13.8v individually rather than rely on a single 24v battery charging voltage supply of 27.6 volts.

One way to achieve this without removing the batteries from the scooter in order to connect them to their own individual 13.8v chargers is to replace the two pole charging socket on the scooter with a three pole socket so that the isolated outputs from a pair of 13.8v chargers can be wired in series to utilise the extra pole of the plug and socket to connect the battery link wire to the charger link wire circuit.

Not only will this ensure a more balanced charge between the batteries it also, and more importantly, reduces the risk of a bad battery destroying a good battery from overcharging. It still doesn't address the issue of cell imbalance accelerating the wear and tear on the remaining good cells in an individual 12v battery but it's one less risk to contend with.

[1] Of prime importance with regard to cell matching in this case is the self discharge and ageing characteristic. Capacity balance whilst still important is of lesser concern here, provided the mismatch is within the wider tolerance allowed compared to the self discharge and ageing characteristics.

[Johnny B Good]

Bad advice I'm afraid when it comes to SLAs where such gassing charges will damage the cells. It's why the SLA battery manufacturers recommend a current limited 13.8v constant voltage charger per 6 cell battery.