Is over charging a gel battery OK in order to get a resting state full charge

[Donald Siegel]

I have two Renology 12v 200 amp gel batteries in series used with a 24v MPPSolar all-in one. The battery spec sheet shows a full charge at 12.8v or 25.6v for my two in series. They recommend to discharge to 50% or 12.1v/ 24.2v for my batteries. I have a hall effect meter. While bulk charging the inverter and the hall effect meter agree pretty well and I will eventually get a "full charge" at 25.6v. If I stop there at a hypothetically full charge, the voltage predictably sags back once the battery is disconnected and rested. It will actually sag back to 0.8-0.9v lower than the peak voltage observed while charging. To get a resting battery voltage and a full charge of 25.6v I need to bulk charge to 26.4 or 26.5v on my two volt meters. The questions. Is it a poor practice to over-charge a battery to 26.5v to get it to a full charge of 25.5v? Is charge voltage overshoot actually over-charging, or are voltage readings while charging invalid? Is a 0.8-0.9v difference in the inverter, and hall effect meters voltage reading while under charge and the final resting charge defined as the battery disconnected and rested 12 hours too high a spread, and a red flag that my new batteries have a problem? I cap my charging at 20Amps, the PIP usually is charging between 8-15amps. Charging for this discussion was on grid.

 

[gnubie]

If you check the specs for the battery you will probably find the float voltage is actually 13.6/7/8 volts so charging to 26.6 is actually not actually bring them up to full charge. If you check the specs on the web page cyclic use charging is 14.2, ie 28.4 for both in series. This would be the absorption stage charge voltage.

One thing be exceptionally careful with when charging gel batteries in series is that they both maintain the same voltage. If they batteries have drifted away from each other in voltage (state of charge) or just have different capacities for some reason you will see the voltage on one battery be say 12.8 volts and the other 15.6. 15.6 will cause the rapid demise of a gel battery by splitting the electrolyte, causing gas bubbles in the gel and reducing plate <> acid contact.

You also need to check the MPP charger settings to ensure it is configured for gel, ie to never attempt equalisation charging. I don't know if that is actually configurable for the MPP chargers, but I'm sure someone here that is familiar with them will speak up.

If you don't have a balancer or other thing to ensure the voltage across the batteries remains close you should IMO seriously consider doing that.

*edit to include more information*
The voltages you see while charging can't be used to refer to resting state of charge voltages so when you see 26.5 under charge, disconnect and let the batteries sit for a few hours it's perfectly normal for the voltage to drift down. After a couple of hours with no load and no charging the voltage you measure on the terminals can be used as an indicator of state of charge.

 

[offgriddle]

EGADS!!! With GEL batteries the electrolyte, (acid), is suspended in a gel, which is why you can turn the battery upside down without the acid leaking out. If you overcharge a GEL cell, the GEL drys out and you've ruined the battery because unlike flooded lead acid batteries, you can't add electrolyte, (aka hydrochloric acid [or distilled water which becomes acidic]), to replace what's boiled off due to overcharging or the routine of run cycles. Side Note: A GEL battery and an AGM battery and a FLA battery are all lead acid batteries.

 

[Donald Siegel]

I got those fully charged voltage numbers of 25.6v from the Renology web site (attached). The specs for battery itself does state a charging voltage of 28.2v I thought that meant charge voltage must be higher than the battery voltage but did not understand that you could charge to 28.6v. So if a charge voltage for a battery is listed at 28.2v as mine is you then let the charge get to 28.2v then it settles back to the float voltage and is maintained at the float setting? The MPP does have an “AGM” battery setting it also has a user defined setting. I did take these new batteries and had them in parallel to get them exactly equal in voltage before connecting in serial, and will check that from time to time to insure they stay that way.

 

[gnubie]

Usually the charger will take care of the details in 3 steps. Bulk charging until it sees the current tailing off, then change to higher voltage absorption charging (14.2 for your gels) to increase the charge rate to account for lead acid's peculiarities, and then once the current tails off to a low value, fall back to float. Integrated items like the MPP should get it right as they can (or should) sense their own load on the DC side and take that into consideration when determining if absorption charging is done.

People using external chargers with a load direct on the battery often fall into the trap where the charger senses the load current and never exits absorption charging silently killing the battery. Quality chargers include a maximum absorption charge time as an option if loads are placed direct across the battery without a current sensing device so that the charger can see what is really going into the battery.

 

[offgriddle]

Usually the charger will take care of the details in 3 steps. Bulk charging until it sees the current tailing off, then change to higher voltage absorption charging (14.2 for your gels) to increase the charge rate to account for lead acid's peculiarities, and then once the current tails off to a low value, fall back to float. Integrated items like the MPP should get it right as they can (or should) sense their own load on the DC side and take that into consideration when determining if absorption charging is done.

People using external chargers with a load direct on the battery often fall into the trap where the charger senses the load current and never exits absorption charging silently killing the battery. Quality chargers include a maximum absorption charge time as an option if loads are placed direct across the battery without a current sensing device so that the charger can see what is really going into the battery.

"Dumb Chargers", the silent killer of batteries sucking the life out of them, very grave indeed ....?‍

 

[grizzzman]

One thing to remember is "ya got pay the bank" Simply put, You need to put in more then you take out. Anywhere between 6 to 15% (more if a battery is aged) FLA