Adjusting battery charging according to temperature

[martin007]

Hello.

Adjusting battery charging (Gel and AGM) according to temperature.
It is usually claimed that the charging voltage for gel batteries at 20º temperature is at a maximum of 28.2 v
28.8 in AGM.
Some manufacturers may have somewhat different recommendations.

For each degree of difference in temperature is compensated by increasing or decreasing the voltage by 0.005 v per cell and degree ºC


Does this apply "Always"to any AGM and Gel battery manufacturer?

[Burgerman]

More or less yes. And its battery temperature not room temperature. So its ard to do it accurately.

But realise that if you just drove around for hours your battery will be warmer than your room/chair by a few degrees. And the act of charging them warms them up. And the act of charging at too high voltage warms them up some more! So that you eventually get a runaway and the batteries melt.

If in doubt its always safe to lower the voltage a bit. For GEL batteries for e.g the recommended charge voltage at room temperature (say 20C) is 13.8V to an absolute max of 14.1V (28.2). So remembering the battery may be a few degrees warmer than the room, you would be better to choose less than 14.1V per battery. But if you were to choose say the lower figure, charging times are extended to days... Instead of 12 to 16 hours. So its a matter of balance.

You are going to need the chair when you wake up? then choose 14.1V (28.2) and you should be OK and it will charge as fast as its possible to do without damage.
If you choose a lower voltage then its definitely safer, say 28V or even 27.8V. But realise that this will take about 8 hours longer.

Its important to understand why.
Once a battery is fully charged or almost so it begins to produce oxygen and hydrogen bubbles. This can be RECOMBINED back into water as long as charge current is low - below maybe .5A on a wheelchair sized battery. So if the charge voltage is too high, this current is also higher and the battery cannot recombine the gasses back into water fast enough. And this process makes heat and wastes energy. So the now warmer battery needs an even lower voltage, but doesent get one... So the gassing/amps all increase. The reason a gel battery must use a lower voltage is because these bubble damage the gel which dries out and creates cracks and voids. So no longer touched the plates surface fully. And it doesent recover well.

As for temperature MK and oddysey say this for gel and for AGM batts:

[martin007]

OK.

[Burgerman]

If we really want to get the best cycle life from a lead battery of all types we would charge them at their best float value. So say 13.45 on a gel battery. 13.6V on an AGM battery. But that would take around 3 to 4 days to fully saturate the cells. To reach a completely 100% charged state. This is because these voltages are low enough that there is no grid corrosion caused or gassing involved at all. So these will prolong battery life and cycle life. But we dont have days to charge.

Its important to get to 100% charged because otherwise some lead sulfate remains on the surface of the plates. And inside of the lead "paste" that is made of finely ground lead particles that is pasted inside of the plate. If we remove a battery from charge before its 101% fully saturated with charge then some of this sulfate remains instead of being returned back to the acid/electrolyte. And it builds up every single charge. The problem is that in time, esp when warm, it turns into larger crystals that no longer conduct electricity. At this point they are there forever which reduces capacity and increases internal resistance. Both of which make charging harder, and slower, and reduces range.

So in order to get a good (but not complete) charge overnight as thats all the time we have. We must force the energy back in faster. The way we do that is to use more volts. And it doesent take much! So with 20C room you would use 14.7V (29.4) with a quality ODYSSEY type battery. And 14.4 to 14.7V depending on the manufacturer spec for normal AGM. And 14.1V (28.2) for GEL batteries. We cannot keep doing this for many many hours however like we can at float voltages, as it will cause excessive gassing and grid corrosion issues. So we generally do this for a max of 4 hours if a battery is almost full. Up to 12 hours for a battery that has been deeply discharged. MAXIMUM. 8 hours is usually the best CV voltage maximum general setting.

Or until the charge current has stopped falling over say 1 hour period. Or end charge at 1000th of capacity - so 74mA plus 20mA for the drain current of the OFF wheelchair on the MK 74Ah battery. Whichever occurs first. Or we stop charge when charge current is still around .5A and then go to a lower float voltage. This usually results in a battery that is as full as we can get in the 8 to 10 hours overnight. Probably 99%. But that leaves that 1% to 2% which means that theres still some deep lead sulfate inside the lead paste in the grids. We can use the chair. Get 99% range. Its the best we can do in the time we have available. But we should leave the chair on a float charge for a couple of days at least 2 or 3 times per month if possible. Every day you get a chance! To return that lead sulfate back to the acid from where it came. Theres no quick way.

Correct charging, and correct accurate charge voltages, along with leaving the battery on charge after the green light on float as often as you can is the difference between all those complaining that these batteries are not what they used to be, or replaving them frequently or other battery issues. And long life reliable performance. So like the vast majority, you dont have to bother! Anything will charge them! In a fashion. But those that understand dont keep throwing them away! A little knowledge is as always power.

[martin007]

Knowledge is power.

[martin007]

Does the floating voltage also have to be adjusted (0.005) according to the temperature?

[ex-Gooserider]

Does the floating voltage also have to be adjusted (0.005) according to the temperature?

In theory it should, it probably won't make a lot of difference as long as it isn't to high.

I'd also note that the frequently suggested idea of plugging in for even a brief 'top-up' charge at any reasonable opportunity will keep the batteries closer to being fully charged, so that when you plug in for overnight the battery will spend less time in getting to the 99% level, and have more time on 'float' where it can get closer to the full 101% charge that we want...

(Also it is worth noting in the tech manuals that the less you run the batteries down in a given charge / discharge cycle the more cycles you will get....)

ex-Gooserider

[martin007]

(Also it is worth noting in the tech manuals that the less you run the batteries down in a given charge / discharge cycle the more cycles you will get....)

I´m aware of it.

[martin007]

Does the floating voltage also have to be adjusted (0.005) according to the temperature?

What do you think, Burgerman?

[Burgerman]

I think at the temperatures our room and houses are at, then if you just settle for the 20C room (25C battery) temperatures then it is good enough. If in spain at 35C I would drop it by about .2v in summer.

[martin007]

I understand that.
What I say is that if the charging voltage is adjusted according to the temperature (of the batteries), the float voltage should also be adjusted.

[Burgerman]

Yes. When its cold the charge and float voltages need to be a little higher. My van in on float right now outside. Its -2 centigrade. So charging at 13.6 instead of 13.4 long term float.

[martin007]

OK.


0.005 X - 1ºC X number celules

Thanks.