Battery Charging Amp Question from Beginner

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eas1906

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Joined
Jun 22, 2021
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5
Greetings,

I'm a newbie in working with lithium batteries and I'm hoping that your collective expertise can assist in answering my question. My apologies in advance if I have posted this to the wrong section and if so, please move me to the correct section.

I built a 3300W solar generator and recently upgrade the batteries from a 50Ah AGM to 40 14.4v 6.6ah 95.04wh Li-Ion Module with BMS from BatteryHookup.com to give me 3802Wh of power. Each of the 40 batteries consists of a 4s3p 14.4v 6.6ah already made battery pack each with a working BMS. I have connected the batteries into 3 strings consisting of parallel connections for each of them with 14, 14, and 12 batteries respectively (see pictures) and then all three strings are parallel connected to give me an overall battery pack of 14.4v, 264 Ah, 3802 Wh (which at the time was a steal for $280!).

I have a 30A solar controller that I will be using with 300W of solar panels for solar charging and I am planning to also connect a 20 A Drok Adjustable Voltage DC Power Supply to the PV inputs on the solar controller to charge the batteries with AC power for regular charging.

BatteryHookup.com recommends keeping the charge and discharge limited to 3 amps per pack or 0.5C. So my question is, given that there are 40 of the individual battery packs each with their own BMS and that all 40 packs are parallel connected, what is the recommended amperage I can push from the Drok power supply through the 30A solar controller to the collective string of batteries? I was thinking of pushing 20A into it since it would be spread across the 40 individual packs, thus keeping with the .5c recommended by BatteryHookup.com. The current would be pushed to the batteries via a 30A fused connection with 10 gauge wiring. Is my thinking correct? Any advice you can provide would be greatly appreciated.

Sincerely,

eas1906
 

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daromer

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Oct 8, 2016
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5,659
Should be fine. Just beware of what happen if 1 bms shut down... Or 2 .. Or what happens when you push it harder.

Having multiple small bms systems like that in parallel works fine as long as you stay well below its max rating.

Go for it!
 

italianuser

Member
Joined
Feb 25, 2020
Messages
340
Hi there Eas, welcome.

Wow, first time I see a configuration of this kind.

To connect batteries in parallel you should have same capacity for each battery, so instead of 14+14+12 I would make a 13+13+13 and leave one out of the pack.

Yes 100% for fusing the main connection to the entire battery storage with 30A fuses on both negative and positive plus a DC MCB which protects against short circuits.

Did you already switch it on?
 

eas1906

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Joined
Jun 22, 2021
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5
Hi Italianuser,

Thanks for the feedback. Yes, I've switched it on and powered various things in the generator including 12v appliances (e.g., LED lights, USB chargers, etc.) and both of my inverters (3000W modified sine wave & 300W pure sine wave) with AC appliances hooked up to them with no problems. I'm in the process of discharging the batteries to prepare for charging (which I have yet to do). I hadn't considered the implication of the 3 battery strings each having different capacities. I like your suggestion of making them all 13 and taking one off! That should be easy enough to do.

I will look into the DC MCBs. Thanks for the suggestion. I admit I hadn't considered putting fuses on both the positive and the negative. Is that standard? I'm using a 12v DC fuse block with fusing on the positive and none on the negative bus bar.

Sincerely,

EAS
 

italianuser

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Joined
Feb 25, 2020
Messages
340
Nice to know it works good! (y)

It's ok to fuse a single pole, it's a closed circuit so it must pass by both poles. I doesn't harm, what should happen if you fuse both poles is that one will blow before the other, I suppose it depends on fuse tolerance, one might blow at 30.4A and one at 29.9A. An extra safety measure, real cheap, makes me feel safer. If it should blow and I must buy another one, can happen, I'm covered in any case.

MCB, I buy TOMZN ones, they aren't expensive but they have different certifications (and CE marking, important in Italy): https://www.aliexpress.com/item/1005001441469313.html

1624486911336.png
 

eas1906

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Jun 22, 2021
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5
Nice to know it works good! (y)

It's ok to fuse a single pole, it's a closed circuit so it must pass by both poles. I doesn't harm, what should happen if you fuse both poles is that one will blow before the other, I suppose it depends on fuse tolerance, one might blow at 30.4A and one at 29.9A. An extra safety measure, real cheap, makes me feel safer. If it should blow and I must buy another one, can happen, I'm covered in any case.

MCB, I buy TOMZN ones, they aren't expensive but they have different certifications (and CE marking, important in Italy): https://www.aliexpress.com/item/1005001441469313.html

View attachment 25481
I have a 30A circuit breaker laying around that I think I can use. This would do, correct?
 

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italianuser

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Feb 25, 2020
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I have a 30A circuit breaker laying around that I think I can use. This would do, correct?
I don't know that product, if it trips for a current overload and for a short circuit I suppose it's ok.

Edit: I searched it, it's not a circuit breaker, it's a fuse holder and a thermic switch
 

italianuser

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Feb 25, 2020
Messages
340
Uhm, I suppose it's ok. Normally speaking you'd identify a circuit breaker with an MCB (thermal and magnetic overload circuits) or a similar device with some sort of protection again arcs, too. But you can technically call what you bought a circuit breaker LOL :D although I don't think it has both magnetic and thermic protections nor anything against current arcs, should open one to see what's in it.
 

OffGridInTheCity

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Dec 15, 2018
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1,712
I have a 30A circuit breaker laying around that I think I can use. This would do, correct?
I used this style on the input to my iCharger X8 (from a battery) as short protection and on/off. It went bad after maybe 30 on/offs - and caused an intermittent connection. If you use them - maybe test them and don't do on/off very often :)
 

Wolf

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Sep 25, 2018
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1,568
should open one to see what's in it.
Absolutely nothing but a bimetallic spring that gets hot and trips the cam. No arc suppression or anything. I had a bunch of them and actually welded the contacts together on one (250A) when closing the circuit while charging capacitors on an inverter. If it was the only breaker I had access to I would be forced to use it and run a pre-charge resistor circuit. Get a real DC breaker , I use the ABB S202 series breakers. With them you can secure both pos and neg. They can be found used on eBay for reasonable money in all kinds of trip amp specifications with every imaginable trip curve. They are IEC/EN 60898-1, IEC/EN 60947-2, rated for 125V DC and UL 1077 rated for up to 110V DC. Additionally there are accessories such as shunt trip, aux switch, and under voltage release available. Mount easily on a DIN rail
Quick search and found some ABB 2002M series B trip curve 32A on eBay ( which are the same as the S202 but 125V UL spec) for $20.00 make offer and $10.00 shipping. I bet you could buy 2 of them for 30 bucks so for 45 you could have a real breaker. Matter of fact I think I'll put in for 4 of them at 10 bucks a piece and see if he'll bite🎣.
Now wouldn't you rather have this protecting your house and investment:
1624543596986.png1624544262538.png1624545768067.png
Or this? Just asking.........................
Wolf
1624546766978.png
 
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eas1906

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Jun 22, 2021
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You've given me a lot to think about. I've spent a fair bit of time in the solar generator space & from my observations a lot of attention is spent on batteries & building the generator but I've not really seen the level of attention paid to fusing as you all have done. I don't recall ever seeing discussions about fusing criteria to include thermal, mechanical, & magnetic circuits. Thanks again for the insights & MCB info!

Eas
 
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