18650 from AGM

[wheresthesun]

I have a 100Ah AGM. I've been learning a bit about 18650 cells and gathered about 100 decent ones. I've built a small 15Ah 3s pack with a cheap BMS. I'd like to use this as an "addon" to my existing setup.

I'm thinking of connecting the new pack to the AGM as the input for the BMS and limiting the voltage to about 12.6, which should charge the pack. From the pack I will add some very low powered devices, < 2A in total.

I'm hoping someone here can tell me why this is a bad idea! I'd like to solar charge the new pack on the existing setup, if possible. Thoughts?

 

[snoobler]

HORRIFIC IDEA. YOU WILL DESTROY ONE OR BOTH BATTERIES.

MORE LATER.

 

[wheresthesun]

HORRIFIC IDEA. YOU WILL DESTROY ONE OR BOTH BATTERIES.

MORE LATER.

Eek. Doesn't sound good. Lucky I asked so!

 

[snoobler]

Lithium NMC is a very bad choice for 12V. 3S is 9-12.6V usable, 4S is 12-16.8V usable - kinda worthless in a 10.5-14.4V typical application.

In order to fully charge your AGM, you MUST charge it to its absorption voltage (14-14.8V typical) and hold it there for possibly hours.

Failure to keep a 12V AGM/FLA at or near full charge on a consistent bases causes deterioration.

Charging 12V AGM to 12.6V will result in it never being fully charged - likely not much more than 50-60%. This will cause the battery to degrade internally causing rapid desulfation and capacity loss. Its usable life will be dramatically reduced.

Obviously, charging a 3S lithium to > 12.6V will just plain destroy them.

 

[wheresthesun]

Thanks for the reply. Is there any way to add more capacity without adding a battery with the same matching type and capacity?
What if I charge the lithium battery pack via a 3A charger plugged into my inverter. Seems like a long way around but would allow me to solar charge it (in a round about way). Or should I ditch the idea and think of something different use for the pack?

 

[snoobler]

Charging it with a separate charger powered by the inverter is fine. Paralleling it with the AGM is the bad idea.

 

[DThames]

You didn't describe your load setup, nor your charging setup very well. That might help.

Something that I did (related to growing a 18650 bank) was to make parallel packs of cells, into a large "cell", then series these cells. For example using 3x6 plastic cell separators, make 3, 18p packs. You can then series them together, connected to your BMS. At your series connection points include a Y balance cable or a type of bus system. At the power connections, also include some sort of Y or bus connection. At a later date you can add 3 more equal packs (still balanced on one BMS) to had amp hour capacity, not peak amp capacity....which should be designed around the BMS and wiring size.

With your 18650 pack you can power a boost type adjustable charger to keep your AGM supplemented. If your boost charger was set to charge to 13.6v for example, when your AGM was below 13.6v you would be drawing some power from your 18650 pack. I am not sure if this is the type of solution you are looking for. At some point you might want to set the AGM aside if your 18650 packs get large enough.