diy solar

diy solar

Connecting batteries in parallel when #1 is full and #2 is empty

You will not have to worry about charging at high C. First the battery tender only puts out 1A, second the BMS charging only path resistance is set that charge current is very limited even at full absorb level external charging voltage. When BMS does re-engage the battery tender will limit current to about an amp.

If you left it connected for a week or two, the battery tender might take it up to 14.2v before dropping back to 13.2-13.5v. You should probably avoid letting battery charge sit for days at 14+ volts while tender is trying to make it to 14.2v.

Some battery maintainers do not have the 14.2v absorb cycle. They just sit at 13.2-13.5v.

My suggestion was to use the maintenance charger to raise the battery voltage above the lower knee before subjecting it to a high C rate change.
 
I tried that all day with 6 (250 watt) panels. Growatt’s screen is off the whole time. Yet you would expect that that solar charger would still work. It didn’t. Now that I have the second battery I should be able to get it started on solar. While on solar is it relatively safe to swap out the batteries?
Not sure you want to hot swap as much as get the solar charging. When the batteries are fairly close in voltage connect the second battery.

Are you concerned about the surge in current from connecting the batteries? A long piece of smaller wire could be used to equalize the voltage before you connect with some heavy jumpers.
 
Once BMS clears the low cell disconnect you okay to do normal charging.
 
Just a note about dinky battery tenders (many brands) and large batteries:

Many have a safety-timer to restrict overall CV time from anywhere from 12 to say 36 hours or so. They may then drop to float making you think you are fully charged. You are not - the CV safety timer kicked in before you could finish the actual CV stage.

Some will reset the CV timer by power-cycling the dinky thing and starting again, but that's not good practice really.

Moral: don't always trust vehicular-style chargers to act like lab or other power supplies.
 
SOLVED: connected a 250W 24V solar panel directly to the depleted battery. Waited for it to charge from 21VOC to 23VOC (took no more than 10 minutes). Now (for the first time in forever) the battery is able to start the Growatt and I’m charging the rest of it up from utility. So, it will be same voltage as the other battery #1. Then I will be able to safely connect them in parallel.
 

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