One battery really sagging in 36V SLA series set.

[drutledge]

I have 3 35ah SLA batteries wired in series on my hefty cargo trike. Everything has been fine until recently, I have about 500 miles (375? ahs) on the bike/batts now.
After I take the bike off the charger everything runs well for the first 5 miles or so, about 3.5 amp hours of use. All the batts sag about the same under full acceleration, about maybe .5 volts under a 30 amp draw for about 10 seconds. Batts show at least 37 volts combined when at a steady few amp draw when riding at this point.

Problem starts soon after and it seems to happen quite quickly: when accelerating, battery #1 (+) will sag all the way down to 6-7 volts while batts #2 and #3 (-) sag in unison the normal amount. When the throttle is lifted, batt #1 instantly jumps back in line with the others in terms of voltage reading. It's hard to read while riding but batt #1 also seems to 'recharge', ie jump in voltage, more than the others when the regen brakes are active. It does not rise faster than the others though when put on my 36V 5amp charger, it rises pretty much in line with the others.

Any ideas? Batts are wired together with #10 wire. If it was a bad connection I would think it would occur from the get-go not just after a few miles of use. Would a bad cell only show itself once a batt becomes partially drained? Maybe not related but should I be rotating these three batteries much like one would rotate tires?

Thanks for any help, Don

 

[dogman dan]

Sounds out of balance at a minimum, toast at maximum. Do you charge at 36v, or 12v?

 

[Pure]

I agree with dogman, but am leaning more to the toast side of things. It's possible there could be something going on with the wiring, so it won't hurt to swap them around and see if the problem duplicates on a different cell in the same position. Doing so should give you a pretty definitive answer.

 

[Zeropointbug]

I would say it is toast. If you say you have 500 miles on those batteries, then I would say they are ALL pretty well at the end of their life. Time to buy new ones, or upgrade to Lithium.

 

[amberwolf]

That SLA is toast--it has more than one bad cell in it that has really high resistance, whcih is why it sags so much when high currents are pulled, and voltage rises so much when high current is pushed into it.

It might be possible to desulfate it, but it would probably be a lot easier to replace them (assuming you can afford to do so). If you like, there are a number of desulfator circuits you can build from scratch, if you google around, but the results from doing so vary widely.

And as has been pointed out, given the mileage/age, all of your SLA may be undergoing the same process to varying degrees, but the others may be fully usable given the high-capacity/size.


If there is nothing being tapped off of just one of the batteries, and all three are simply wired in series with the bike running only off the two end wires, then there is no need to rotate them.

However, if you have something (like 12V lights) running off of just one of the SLA, and they use up enough power, that SLA will become progressively less-charged at each use, relative to the others, unless you leave them all on the charger a nice long time for them to self-balance (assuming it has a trickle-charge or float-charge state, and doesn't just shut off when it reaches "full" pack voltage).


As dogman asks, how are you charging them? If charging in series, what is the actual charging voltage and current? If charging in parallel, it is possible a higher-resistance battery will never fully charge while other lower-resistance batteries are in circuit--they will absorb most of the charge current, and the already-problematic battery will only get worse.


EDIT: ignore the last part; I see you are charging them in series with a 5A charger. Do you know what the actual charging current is, and the actual charging voltage?

Most likely it is simply a bad SLA battery.

 

[drutledge]

The charger charges at 5 amps to 44.2volts then floats at 40.1. They all seem to charge at a roughly equal rate. If anything, the middle battery is always about 1/2 volt under the two end ones (the #1 or + one is the bad one)

Since the set of batts works fine for a good while before the first battery sags, I was thinking that a wiring problem seemed remote.

THere is no extra draw,lights etc, on any of the individual batteries.

I put desulphator on this weekend while I'm out of town. It's just on the bad battery since its for 12v. I was thinking that if that batt was sulfated that maybe that's why it works fine at first then drops off quickly. Batts sulphate from the bottom of the plates I think I read somewhere. Maybe when the top part of the plates have given all they can the bottom part is too covered to offer much. I bought a desulphator off of eBay, "wizbangplus.com" if I recall correctly, any opinion on their quality? Any other suggestions?

I can't say that I've noticed any performance differences from the other two batteries despite them having 500 miles on them. They still have plenty of 'oomph' and seem to charge well. Don't sag much under heavy load.

Thanks for anymore help and info.

Don

 

[amberwolf]

I was thinking that if that batt was sulfated that maybe that's why it works fine at first then drops off quickly. Batts sulphate from the bottom of the plates I think I read somewhere. Maybe when the top part of the plates have given all they can the bottom part is too covered to offer much.

With flooded lead-acid (FLA), that's probably true, as the sulfate crystals fall to the bottom of the electrolyte.

But with sealed-lead-acid (SLA), they usually have gel rather than liquid, so the sulfates form in-place, and stay where they started.

However, the rest of the theory is probably sound. I don't really know for sure on the mechanics of the process and how it affects power delivery, only that it does.

I bought a desulphator off of eBay, "wizbangplus.com" if I recall correctly, any opinion on their quality? Any other suggestions?

No, I've never used any of them, only read of many different instances of various types; results vary so much even with the same batteries and same desulfator, it's really hard to say if any particular instance will work or not, without just trying it.

 

[dogman dan]

Nothing to lose by trying the desulphator. To really find out if that battery is lower capacity, and measure any improvement, you could do a crude capacity test. Use a 12v headlight or something to discharge the battery, and graph it's voltage over time. See if it comes back any.

It could be both, a bit more sulphited than the other two, AND undercharging. As the pack ages, maybe do some 12v charging periodically to make sure each one is really topped up completely. Then you know the pack is as balanced as possible.

500 miles shouldn't be too worn out, but it could be, if you discharge too deeply.

 

[drutledge]

After desulphating for 3 days with usage, I tried the bike out again and still the battery sagged heavily after a few miles. Oh-well.

 

[amberwolf]

Depending on how that DS works, and how badly the battery is, it could take days, weeks, or months to do it's job. Or it might not work at all.

Or it might not be sulfation at all, and instead be lost electrolyte from overheating during use or charging, boiling some of the moisture out--that's really hard to fix in most SLA. Or it could be plate damage, or some other defect.