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Question about Lithium Ion Battery and protection, charging

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Hi Guys,

I have recently found out that my source for $70 70AH 2nd hand deep cycle batteries is not really as cheap as I thought! haha, I never like using the 2nd hand, but its what I can afford!
considering if I DIY Lithium Ion I might come close to the same price with respects to discharging my deep cycles to around 50%, captivity price wise would be similar and I would get 100% brand new!

I have read a few articles here and there and am a little scared with respects to charging them and how to go about it.. considering I am managing 2 strings of 6 ( 12 batteries with 2 lots of 6 batteries in Series for 24v...)
1 Deep cycle battery costs me $70 is's a 12v x 70A = 840Wh (40-50% (336-420 Watt Hours))
so speaking about capacity my banks total is
12 x 70A x 12v = 840Ah (approx 10 KwH)
with my level of discharge at 40-50% or so.. thats only 336-420Ah, (4-5 KwH) of usable power

so I have seen decent batteries for sale at 40 cells of 18650 for $60 + costs for circuits and holders etc
mathematically that is 4.2v * 3Ah * 40cells (504 Watt hours) vs the 336-420Watt Hours the deepcycle are doing
that would mean 40 cells x 10!! for my old capacity! 400 batteries! OMFG tho it's make expansions over time as building the batteries will take time haha

so you can see my excitement here with respects to high efficiency batteries, and using these cheap 18650 cells over brand new deep cycle batteries
we're talking $60 for 18650s vs $200 for new deep cycles,

with that out of the way comes the hard part.. how to I use the cells?
I would want 24v nominal! meaning 6 series cells 3.7-4.2 (22.2-25.2V)

I have seen a few battery protection circuits, and here is where I am really lost on connecting these!

this is a 15A 6cell circuit, I own one of this circuit already but have yet to use it..


now I have seen this following circuit which is way more exciting that it has a discharge terminal and separate charging terminal.. meaning possibly I can connect a few dc-dc converters to charge the strings independently?! tho I am unsure here, I would assume if I ran 4 of these protection boards, would I be able to have the outputs connected together?

this is a 50AMP output/ 15A Input circuit meaning I can make less battery packs if that makes sense


If I am thinking about this in the wrong way I hope to learn the right way, please make comments and advise for my possible new project!
Many Thanks

EDIT: Sorry, I use the power for a small solar powered bungalow


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Most Helpful Member
I've seen 18650s that are supposed to be 9,900 mAH - yes, nearly 10 AH. When checked they are around 700 mAH. The batteries sold on ebay tend to be rebadged and nowhere near the capacity stated. Caveat Emptor.

Protection should be incorporated into the battery itself. Trying to do it yourself is full of problems. These batteries on Alibaba have the Battery Management System (BMS) built in but they're not cheap - I assume the $50 is for a small capacity battery. However, the Chinese are very flexible in negotiations. You can also get arrays of 18650s such as these with BMS.



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The 7 cell charger/protection circuit does not "balance charge" so the weakest cell will overcharge and blow up or catch on fire.


Hi guys,

thanks for the replies, I know about the dodgy vendors selling the fake 10Ah cells, the the flooded comments.. 700mAh haha,
I have got a few cells from a seller saying 2900mAh and they were.. and they weight around 50g each I have researched this and the seller seems good.
have had these cells for months now and seem very good..

So I want to give some info on my charge system, so far I have around 1.5KW of solar panels, and the deep cycles get approx 45-55A @24v

So I have been thinking.. I would want to try to get 1 Battery Protection Module that can handle everything without the need to require multiple..
but they usually come in 3 cell and not 6cell, they also have low charge current. so i might be stuck with getting a few smaller ones :(

would using 5-6 pcs of
and joining the 5-6 outputs, to make 1 big battery bank, would it work?

The BPM above is rated at 15A charge/discharge so 5-6 pcs would allow for a max of 75-90A charge/discharge
10 pcs would be 150A

I like how the diagram shows the battery and outputs are isolated. would the packs be safe for example if a few cells went bad.
so if internal voltages go bad the BPM will cut off that pack.. and hopefully protect everything?
like a daisy chain, if one pack goes, than a second and third packs go wrong, it the amps being drawn is too much it would detect over current and shutdown.?

If I can find large AMP modules and bring the count to about 5 Units.. such as 30A Discharge / 15A charge
If all the 5 Battery packs outputs were joined, can I simply charge by that same connection?

* ? * Is it a safe and common thing to do? joining battery protection modules?
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