Replacement of Lead Acid by 18650 LiCo

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ThomasR

New member
Joined
Nov 13, 2021
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1
Recently I had to replace a 12V 0,8Ah lead acid battery on the fly to make the gadget work again ASAP. The only parts at hand were some old 18650s salvaged from laptop computers. They were all tested and in good condition and were already holding up to 4.11 Volts for weeks.
So I quickly welded a triplet with a matching BMS and made somebody really happy with that instant solution.

Now I’m wondering what the long term consequences might be. The lead acid battery was already some 6 years old and only used to support the external power supply. The gadget includes a small stepper motor and a more power hungry wiper motor to drive a small door. I guess the load spikes of the motors would stress the external power supply and need a local booster.

The lead acid says: constant voltage charge, cycle use 14.5 - 14.9 V, standby use 13.6 - 13.8 V. Given the nature of the design, the charging system will now most likely deliver continuous 13.8 V to the LiCo BMS.

The BMS will prevent any overcharging and limit the internal voltage to 12.6 which the gadget was completely happy with. Since there is continuous external power supply, the LiCo will stay fully charged forever and should not have any issue??

Please comment, thank you!
 

OffGridInTheCity

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Dec 15, 2018
Messages
1,862
Let me preface with... This is a DIY channel, and I don't say this as you must! - too much of that these days - but just offering an honest reaction :)

.....
The lead acid says: constant voltage charge, cycle use 14.5 - 14.9 V,
*If I'm misunderstanding - then OK but seems like 14.5/3 = 4.8v/cell charging could occur.
If the BMS fails, sounds like the cells could be hit with a sustained overcharge - which means FIRE!!

I'm not getting the actual the voltage range of operation - but can you do 4s? This would take care of the overcharge risk - e.g. max of 14.9/4 = 3.725v / cell.

standby use 13.6 - 13.8 V. Given the nature of the design, the charging system will now most likely deliver continuous 13.8 V to the LiCo BMS.
Most likely? - not a good enough bet for me personally due to the above. Can you test/verify the actual situation?

The BMS will prevent any overcharging and limit the internal voltage to 12.6 which the gadget was completely happy with. Since there is continuous external power supply, the LiCo will stay fully charged forever and should not have any issue??
A BMS (to me) is more like "I depend on it to avoid damage to the batteries in limited occurrence / catastrophic events" rather than "I depend on it minute by minute of every day to avoid likely fire".

Please comment, thank you!
In addition to 4s 18650, there are many cheap 12v@8ah LifePo4s - Here's a quick Amazon search showing $45 upwards - https://www.amazon.com/s?k=12v+8ah+LifepO4&ref=nb_sb_noss_2

This year, I started going cheap LifePo4 with things like the generator battery and camper toilet battery... and its been working well.
 
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