Mixing Different Pack Chemistries in Parallel

[tongxinpete]

I have been told it is possible to do the above using schlotty diodes to prevent one pack 'back bleeding' into the other. I can see it working with two packs of the same Ah & cell type. Otherwise I think the pack with the lowest internal res would tend to discharge first, but also one pack having a greater HVC than the other would have a bearing. I am asking if 10 Ah lipo4 could work with 8 Ah Li - ion (Panasonic 18650PF cells). At present each pack would run its own motor (two motors on the bike) I fear the Tonxgin one day giving up & having to go to a single Q128H (poss to replace the Tonxgin with another small front motor) - but there are weight & cost advantages in eventually going to a single motor
It comes down to weather to build a 3P or 4P pack from the 18650 cells. The 4P pack would cost more but would be 'future proof' Both packs are Nominal 36V (actually 43.8V & 44.2V) Reason for going to 18650's is affordable availability in Europe, so cheap surface shipping.

 

[Deleted member 4366]

Yes, you can put them in parallel. I've done it without diodes. I still think that you don't need diodes.

There's one problem that I ignored when I did it. LiFePO4 is charged to 43.8v and li-ion to 42.0. They need to be at approximately the same voltage when connecting them, but the charge difference between 42.0v and 43.8v. In a LiFePO4 is nearly zero, so it should be OK to connect them at those voltages. I left mine permanently connected, but that means that the one you charge, charges the other one through the discharge leads. This bypasses the charge fontrol FET, so, you could get problems if it goes out of balance.

Ideally, therefore, you should disconnect them for charging and check the voltage before reconnecting. Diodes in the ouput leads would prevent that, but that and allow you to charge them separately while connected, but then they might not be at the same voltage..

 

[tongxinpete]

Thanks Dave for the above advice, I will be charging each battpack individually as either will be removable from the bike (security & ability to use a batt across several bikes). I will go with 3P on the 18650 build (cost, & the fact that it is only my second pack build, first with this type of cell)
Another reason for individual charging of the packs is that the proposed cell type has a min & max recommended charge rate, this across 3P falls perfectly within the range of a basic low cost li ion charger. Original goal was a low cost batt build to replace 12 Ah SLA bricks for 15A & less motors. Cost should come in at about £115 all in (even using tagged cells for rebuildability)
SLA would be £70 delivered, but the potentially greater no of cycles alone balance this extra cost, without the benefits of better c rate & much lighter weight
Dependent on how hot the cells run I will house them in either a small cheap tool box on a front rack, or maybe a triangle bag if I can find a budget one (The Li Po4 being on the rear) My mistake quoting 44.2V! I was very tired