Sulfated Greensavers?

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davcr
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Sulfated Greensavers?

Hi all,

I recently bought a 1500W moped that uses four Greensaver silicone batteries (rated 12v 38Ah) connected in series. The customs documents reveal that the moped was actually imported in 2006, so it must have been gathering dust in a store for more than two years. I had doubts about the health of the batteries, so I decided to test the moped at steady 10-12A (about 30-35km/h) one day to see how far it could go, until the batteries were unable to keep above the cutoff voltage of 42V at that rate of discharge. It ran for about 55km (as measured by the odometer) or 1h45min. I assumed that, at that point, the batteries would be almost empty, but, to my surprise, the charger did not feed more than 18Ah into the batteries to top them up again. That meant that, with more than half of the charge inside, the string of batteries was incapable of supplying more than 10-12A, which rendered them completely unusable.

So I tested the voltages of the batteries, and this is what I found
- After running at full throttle in dense traffic (continuous starts and stops) for as much as the moped can run (a disappointing 18 km or 12 mi) the open-circuit voltages in the batteries read something like 12.3V 12.4V 12.4V 10.4V(!)

- However, if I let the batteries rest, the voltage of the weak one steadily climbs up until it equates those of the others.

This sounded to me like the battery might be sulfated. However, for what I've read about sulfation in the internet, the typical behavior of a sulfated battery is just that its open-circuit voltage is too low. And well, this one shows low voltage only right after drawing a lot of current from it, and then its voltage gradually increases until it matches the expected one for the amount of charge it still has inside. I thought this gradual recovery of voltage was related to the dissolution of tiny bubbles of hydrogen that form next to the electrodes because of the fast chemical reaction when a lot of current is drawn out of the battery, and not to sulfation.

So, my question is, is it more likely that hydrogen bubbles form on the surface of sulfated batteries plates than on clean ones? That is, may a sulfated battery behave like this:
- Normal open-circuit voltage at rest
- Lower than normal closed-circuit voltage for large current (this is expected because of the increased internal resistance)
- Lower than normal open-circuit voltage only right after having drawn a large current, that gradually recovers in the minutes time scale (is this expected too?)

What do you think?

And then, I've noticed that the Greensaver charger that comes with these batteries has a final stage of pulsing current, short pulses of about 3.5A at a 1Hz frequency. May this be intended for the desulfation of the batteries? Or is the frequency too low? It's hard to me to check how long each pulse lasts, since I don't have an oscilloscope available (just a plain multimeter). However, the typical frequencies of the desulfators that people talk about in forums seem to be around 1kHz.

Do you think a desulfator might be of some use here? Does anyone know if they are useful for silicone gel batteries as they are for flooded batteries?

Has anyone tried to recover a sulfated silicone battery or one stored for a long time?

Thank you,

davcr

antiscab
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Last seen: 9 months 4 days ago
Joined: Saturday, July 7, 2007 - 23:55
Points: 1686
Re: Sulfated Greensavers?

one of the cells in the battery that read 10.4v has been reversed (probably more than once, each time the scooter has cut out)
it will show significantly reduced capacity compared to the rest of the pack.

it will show significantly higher internal resistance.
immediately after a reversal event, the battery will show lower voltage, but will improve to approximately where it should be.
This is because soem areas of the cell have been reversed, and some still have charge.
The charged areas of the cell will try to correct the revered areas of the cell (and thus show an improvement of no load voltage over time).

Theres no "cure" for cell reversal (aside from replacement).

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km

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