Strange MK Gel battery behavior

[thetascott]

New MK gel group 24 batteries were installed yesterday. I use a Victron charger. Woke up this morning and noticed current was still flowing at 0.2A with the voltage at 27.4V. I then increased the voltage to 28.1V and the current went to 0.8A. I left it alone for an hour to see if the current would fall as it should. After an hour, the current did not drop at all (still 0.8A so less than a 0.1 drop in an hour) so reduced the voltage back down and did a couple tasks before hopping into the chair.

After using the chair for 4 hours today, I checked the voltage using a multimeter and the reading was 26.8V; that seems WAY high to me as the surface charge should be used up.

Is it possible one of the batteries has a cell that never stops taking current?

I wonder if the charger that came with my chair would even switch to float! The current would be higher with 28.8V by quite a bit. Another reason to use a charger that shows what's going on; can catch issues early.

Will be keeping an eye on it tonight.

[Burgerman]

One of he reasons that its hard to sell a decent gel charger (and sometimes AGM chargers) is that the current that they will drop to and the speed that it happens is extremely varied.

If the battery (not yours but any) has been stored a lng time at a less than 101% state of charge then there will be at least some lead sulfate deep inside the pasted lead in the centre if the plates (which are grids with lead particles as a past inside it). It may then take say a 24 hour or sometimes 48 hour time sat at a lower voltage like 13.6V for this lead sulfate to be returned back to the electrolyte. Sometimes it may take a week. Or 10 cycles. Or it may never do so.

And these gel batteries are very temperature sensitive. If the voltage is a fraction too high for the battery INTERNAL temperature then the current wont drop.

This is why we have an 8 hour CV limit when charged at 14.1V and room temperature (20C). 68F? In case it actually increases!

Also if the battery has been on charge a while the core temperature may be a few degrees above the room temperature. Especially if i wasnt dropping fast.

Remember that the BEST charge voltage for MK Gel is actually between 13.8 and 14.1V @ at 20C. So 14.1v is already of the highest side of correct. We do that as its about 2 hours faster at CV than say 14.0V. And about 6 hours faster than 13.8V. And we only have overnight.

Another thing to note is that recombination of gasses that happens at 14.1V produces heat. If its a little sulfated - even a fraction - then it will remain at around 1 or 2A for a long time. And that warms the battery innards through recombination - which is where your current is going. Which means that the voltage is now higher than it could be for the now internal raised temperature, and so it draws more current... Rinse and repeat. If this process continues then eventually the battery will start drawing more current not less. Which makes more gas. Which causes more heat from recombination, and so draws more current and on and on. Thats called thermal runaway.

So use it, then recharge it, but drop the voltage a little to say 13.95V for CV and as soon as the current stops falling by (as MK state) 0.1A per hour then make it drop to a lower 13.6V float. And leave it doing that as long as you can afford. Even up to a week is safe.

It should eventually start dropping to a low current in around 8 to 10 hours CV (not total time) or around 200mA or less though. After a little use. But they do vary. Depends of its charge and usage history, and temperature etc.

[thetascott]

Really appreciate the feedback. If I am understanding correctly, the current issue may fix itself with a bit of patience.

The room temperature was 66-67F overnight in the room. I live in Minnesota, USA. It's warming up outside but have not run the furnace much the past three weeks. Sick of paying for heat and let it get a bit chilly indoors.

I charged at 27.8V for 8 hours last night. Woke up 4 minutes after it switched to float at 27.4V. I did the one hour at 28.1V at this point. I used 27.8V CV because these are fresh batteries and knew the SOC was likely not the same so decided to be gentle on them to start with. Based on your feedback, I will continue to be gentle on them for the next few days.

[Burgerman]

Use normally, charge as above, drop it to 14.0V CV. And let them sit at 13.6V for a day, a week... It will do 2 things. Completely remove any reaining sulfates back to the electrolyte. And make sure that any cells that are at a lower level than the rest catch up completely after being stored. That happens because every cell in a battery has a slighty different self discharge rate. So storing a long time means that some get lower than others. Thats one reason that they should be topped up with a slght overcharge (which is what a 13.6V float is) for 24 hours or so for long periods. Its just enough above the 100% full point that they get a very slow and low current pull up which balances them. Its low enough that no damage or gassing can cause issues. And high enough that the sulfated lead particles and the low cells all catch up. But it takes time. Because the current IS so low at this safe voltage it takes a very long time.

Once its all been cycled 20 or 30 times and charged in use then all should be good. At least as good as its ever going to be.

I no longer have any MKs, but the eternity gel batteries supplied by sunrise behaved the same as yours when new. Now they terminate after 8 hours at CV in cyclic use at around 125mA. And thats the figure I set my PL8 to. They take between 4 and 8 hours depending on either the time limit or the 125mA termination current I set. Depending on state of charge or how deeply they were discharged. Charging in this way they are now over a year old and as new capacity. Not used every day though. I have a few chairs.

[slomobile]

I live in Minnesota, USA. It's warming up outside but have not run the furnace much the past three weeks. Sick of paying for heat and let it get a bit chilly indoors.

I really miss summers in Duluth. Glad I never had to handle those hills in a wheelchair in winter though.