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diy solar

Need help charging my 8S 280 Ah bank

AREA351

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Oct 19, 2020
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I connected them all in parallel to get them all @ 3.3V.

I don't have a bench top power supply and even if I did it would take a long time @ 10 amps for any reasonably priced charger.

My MPP Solar all in one unit has a built in 25 amp inverter/charger.

Can I just plug it in on my bench and let it do it's thing or is a top balance necessary first?

Using an Overkill Solar BMS by the way.

Thanks in advance.20201115_154554.jpg
 
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Considering that you bought budget cells from China, I think you can answer the main question for yourself. There's a reason almost everyone does it.

What did you spend on these? Almost $1000? A $60 power supply and 5 days of charging is too much of a bother?

As I reported in a separate thread, just paralleling cells together doesn't do much. Even left that way for a week, their states of charge will be 10% off, AND they won't be anywhere near the top of the range.

Do you HAVE to? No. Should you? Absolutely.

I recommend you download the top balancing guide from the Resources section.
 
Connect your MPP unit up to the AC and let the built in charger do its thing, it doesnt need solar to work. Get the MPP PC software working to get it set up, that makes it easier. 29.2v Bulk and 27.2v float, no equalization. You have everything you need.
 
I don't mind the 5 days of charging with a $60 10 amp charger, but I don't want to leave it unattended the whole time just in case the magic smoke appears while I'm at work.
 
The Overkill BMS already has built in cell monitoring. I know it only measures voltage but that doesn't really tell the SOC does it?


Screenshot_20201119-231307_xiaoxiang.jpg
 
Here is what I did:

Installed the batteries so they are all strapped and in their semi permanent position.

Attached them to inverter, attached bms, attached active balancer.

I'm using an old server room ups so it only charges at .75a across my 24S battery.

So my install is effectively complete and I never went through any top balancing step. Because I have an active balancer plus low charge rate.

One could do the same with a higher charge rate by charging until one cell gets to 3.4 and setting your max charge at that voltage. Then coming back after a week or two (or a sunny day) and repeating the procedure until all cells are an even 3.4 at max charge.

I'm highly sceptical of the strategy people are using to top balance for a number of reasons. Floating at high voltage, uncompressed cells, building and compressing their pack with fully charged cells etc. This strategy I've worked out for myself doesn't seem to have any of those downsides and while it might take longer, it might be faster.
 
Ignore SOC unless your using a shunt to count in/out Watts, or look at the resting voltage. The BMS SOC value means nothing.
When your MPP charge drops to zero and the charge light goes solid, Its full.
Don't get too freaked out with all this jargon, its just a battery. You have all the pieces in place, go ahead and use it.
 
With the overkill BMS you can charge it to 3.5 per cell and it will shut off if one goes to high on you.
Then you can go from there as o how you want to proceed.
It is the safest BMS I know of.
Would like to get one one day myself actually. :)
 
Would you guys hook up one of those $60 bench top power supplies and leave it unattended for days at a time? I'm in no hurry to top balance the pack, but (8) 280Ah cells is going to take a while and I work for a living.
 
Once you set the target voltage (like 28v is a safe starting point), when the batteries reach that voltage, the current drops to zero. So while its still on, its effectively becomes a trickle charger or its doing nothing. Its a great charger, it shows you accurately everything its doing.
Hope you get the $35 rebate like I did.
 
Once you set the target voltage (like 28v is a safe starting point), when the batteries reach that voltage, the current drops to zero. So while its still on, its effectively becomes a trickle charger or its doing nothing. Its a great charger, it shows you accurately everything its doing.
Hope you get the $35 rebate like I did.
What power supply are you using?
 
Following the guide will be helpful. Series connection first with the BMS attached properly, charge at max until the BMS shuts it down. That will charge the highest capacity/SoC cell to full and get the others close. Parallel top balancing may only take a day or two.

If the PS is properly set to 3.65V before connecting to the battery, I would allow it to charge while unattended provided I could check it at least once every 12 hours, more frequently as able. The instant the CELLS hit 3.65V (they will read notably lower than the PS), terminate charge.
 

Following the guide will be helpful. Series connection first with the BMS attached properly, charge at max until the BMS shuts it down. That will charge the highest capacity/SoC cell to full and get the others close. Parallel top balancing may only take a day or two.

If the PS is properly set to 3.65V before connecting to the battery, I would allow it to charge while unattended provided I could check it at least once every 12 hours, more frequently as able. The instant the CELLS hit 3.65V (they will read notably lower than the PS), terminate charge.
I've already read the guide and watched Will's videos before I posted here.

I guess you guys are saying to top off the cells to about 3.5 volts with the MPP Solar charger and then top them off with the desktop power supply?
 
Please read the specs on lifepo4. They are not that delicate. They can handle temp charges up to 4.2v without damage. If you have a proper charger and a BMS your are good to go. Stop worrying and start using your batteries...
 
I agree with BarkingSpider on the 4.2V limit, BUT this should be avoided.

Run them in series with the BMS set for a 3.65V cut off on the highest cell. If you find that our cells are relatively well balanced, a top balance may not be necessary, but you won't know until you take at least one cell to peak voltage.
 
Took me a long time to top balance my 8s 280Ah cells. I initially hooked them all up in series and charged them with my main charger (29.2v) until the first cell tripped the protection (Overkill solar). Then disassembled the pack and used a small single cell charger until they were all fully charged. Yes, it takes a long time but I don't plan to do it very often. I want to get the most out of my investment.
 
Buy a RC battery charger from a hobby store. Most all of them have mutiple charge profiles as well as balance charging. Connect your all in one solar charger to do the bulk charging your BMS will shut off when the first cell goes too high. Then pull part the pack and top charge the rest of the cells using a hobby charger. Then put the pack back together and carry on.
 
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