Advice on Gel Battery Bank

[Vortechs]

I was lucky enough 10 years ago to buy 8 new Sonnenschein Powerfit Telecom 400, 120AH Gel batteries in an auction for £120.



 I've been using them ever since as a 24v bank for my Micro-hydro. The only time they get run down is when I have to turn the hydro off for maintenance and they've not been run below 24 volts for more than a few minutes ever, they're marketed as having a 12 year life expectancy, but their capacity is starting to go down very fast now. I've always charged them at 27.2V float and 28.2 bulk, they spend most of the time around 27V, but now as soon as I turn off the hydro the voltage drops to about 24.5v within about half an hour while only drawing about 200w. They're sealed batteries, is there anything that can be done to recondition them or am I going to be looking for a new set of batteries in the near future?

[mab]

Well I wouldn't worry too much about 24.5v on an older battery as long as it holds up at that voltage for a long while.

if it really is sinking fast under load, I might first try separating the batteries into separate strings in case it is one or two batteries developing bad cells and pulling the whole bank down.

If they've mostly been 'floating' then they may have lost water from the electrolyte over time, and I've heard about folks opening the gels up and adding water, but I'm doubtful that will help if they're already losing capacity.

other than that I can't honestly suggest any way of restoring old gel batteries, sorry.

[Flux]

You have not done badly, I suspect the 12 year life is for standby use. They may only be expected to see a real discharge once or twice in their lifetime.

As Mab suggests I would check each one to see if it is one rogue or whether the whole lot has lost capacity.

[joestue]

are they Gel batteries or AGM cells

[Vortechs]

Oh well, I guess I've done well with them. They're VRLA Gel batteries. I've tested each cell, all are very close in voltage, there is however a 0.1V difference in the 2 rows of 12V that make up the 24V bank, how do I equalize them?

[joestue]

 i don't believe you can equalize gel cells because the electrolyte is like jello (i don't know actually, never had one to take apart)

agm cells are 98% saturated fiberglass and you can add water to them (but do it slowly), and you can equalize them (which is controlled over charging, the fiber glass gives room for the hydrogen and oxygen to leave the battery. Gel cells don't have any mechanism other than diffusion for the hydrogen and o2 bubbles to leave the battery. so they will expand.. as far as i know, i've never had one.

[Vortechs]

I know you cant equalize Gel's so I've never done it. Could I not try and do it by attaching a 12V bulb to the high side and see if I can bring it down to equal with the other?

[joestue]

so i looked at the company datasheet and to make matters more interesting it appears the gel electrolyte is in fact held in fiberglass mat...

its possible you're losing capacity rapidly because the positive grids are corroding and breaking off the bus bars.

[Vortechs]

So,,, it's an AGM Gel battery?   

[Flux]

I don't know about those ones but usually Sonneschein are gel. There seem to be at least 2 types of VRLA with different voltage requirements. These may be the type with the low voltage, the higher voltage ones which are not gel can be equalised carefully. I don't think the gel can.

You can't do any good as you suggest  with a lamp, you need to raise the low voltage string, not lower the high one as that would just be discharging the better ones.

Possibly the only thing you can do is separate the cells and charge each one with the maximum charge alowed for that type of cell ( should get that from Sonneschein).

This way each cell will see that maximum. In a string the high ones will dominate and reduce the volts available for the low ones.

With other types of battery equalising is done by applying an over voltage to the string, this overcharges the good cells but lets the voltage rise higher on the poor ones to get them back up.

With a string of gels you can't do this as the good ones will not tolerate the over charge.

It is a bit drastic but the only real way to find out what is happening is to do a discharge test on one of the highest voltage cells. If that came out fine you could do the same with the lower ones, if they are poor you could try to improve them by charging individually.

If the load test on the good one showed low capacity then there is little you can do as the whole bank will be tired.

Flux

[Flux]

I have been thinking about this, with hydro and a fixed charging rate you might get the lamp idea to work. If you hold the good cells down to the maximum voltage you can raise the input voltage to bring the low ones up.

This couldn't work with a variable charge rate of solar or wind and it will be tricky with hydro  but may do some good.

[Vortechs]

I remember when I set this system up I wanted a 12v supply to run a 12v fridge and a few other things, I wondered if I could just take a 12v feed from one of the two  halves of the 24v bank but after reading up on it I realized it would cause an imbalance because the controller just sees 24v not the separate levels of the two 12's. This should work because its just doing the same but to correct an imbalance.

[XeonPony]

IMO gells suck, no matter how you cut it, just use them till they're garbage and buy some good flooded ones, or if you feel the need AGM, but given the system I'd use the US2200XS I have had very very good resualts with this battery for over 5 years now with my solar system and auto cell watering system.

[DamonHD]

I don't think that it's fair to say "gels suck"; they're intended for a different style of use than (say) flooded.

I'm very happy with my large gel set (2kWh usable) from which run my ~4W load throughout the winter.

I don't have to worry about topping up, nor my small children coming into contact with battery acid.

Rgds

Damon