Connecting a 2c Lifepo4 in parallel with a 10c pack

[gilfort]

I bought a 50w cyclone motor kit with 8.2 AH 24V Lifepo4 (10C), its instal on a 20" folding bike. It works fine but I did not have enough power so I bought a 24V 20AH V2.5 LiFePO4 Battery Pack from Li Ping in Shangai. This is what he wrote to me before I bought is pack : Actually, we have made many 24v20ah packs for the 500w cyclone motor. We recommend you upgrade the BMS to high rate version that can allow 60A continuous discharge current, because the cyclone motor will draw 50A maximum. Cost to upgrade the BMS is 20usd[/color]. Now I will get this battery with an upgrade BMS. I told LI Ping that I intend to connect the 2 pack together. He sold me 2 diode so the power of one battery will not go from one battery to the other. That is it! Do you think its wil work fine? I know very little about battery but I thought that the 10c 8ah would give me power for acceleration, going up hills and the 2c for continous normal ride. Am I right? Where I live we speak french and their is lot of hills some have 20´denivelation.

 

[dogman dan]

should work fine if you connect up as ping directs to use the diodes. What will happen is the motor will pull power from each battery according to it's internal resistance. So part of it will come from the 10c pack and part from the ping. Since your capacities are very very different, chances are the 10c pack will drain first, but you should be able to limp home on the ping. Just try to go really easy on the throttle on the end of your rides.

I'd think you'd get at least 10 miles before you run out the 10c cells though. And could stop and recharge them if you really need them back to continue the ride.

From what I've heard about cyclones, the amps they draw varies widely depending on the gear selected, if you pedal, etc. So when the first battery goes dead and you are running on just the ping, pedal a bit more in a lower gear and you should be able to continue OK. A wattmeter would help quite a bit to know if you are keeping the amps below 30. I wouldn't advise running a ping much past 1.5c continuous. So for a 20 ah ping, about 30 amps, and better still 20. While the ping is sharing the load with the other battery, the 10c batterys lower internal resistance will keep it providing more of the amps, and the ping won't suffer till it runs out.

I will guess that you will normaly use all 8 ah of the 10c battery, and perhaps a bit less from the ping. So maybe about 12-15 ah of use before the 8 ah battery is drained, then at least 12 more ah use from the ping by itself. So you should have some decent range on both batteries together, and the limp home on just the ping would have just as much range if you ride slow the way back. It won't be as fun to crawl home slow, but you will get there.

 

[gilfort]

I thank you Dogman! My folding bike as 7 gears and even in the 7 its not dificult to ride. Is it not bad to drain the 8ah battery completly? I do not run fast because I use this bike to explore nature in the search of mushrooms to be photograph and sometime be eaten LOL. I am a retired old French-Canadian separatist. I enjoy riding around the country where I live. Since their is a lot of hills I could not go around like that without an eletric bike. I have choosen a 500w cyclone motore because it give me more tracking to go uphill.

 

[dogman dan]

Yes, in general, no matter what chemistry a battery is it is much better to keep the battery from completely driaining. But you should have quite a bit of riding before you actually do drain it 100% with another battery connected paralell with it. Just experiment with it to get a feel for how far you can go before it's totally drained, and un plug it when you need to. Then limp home carefully and low amps on the ping alone. The limp home may be hard on the ping, but if you can keep the amps low it will be ok. So half throttle on the limp home should work fine when going uphill. Down hills should be fine.

 

[gilfort]

I thank again Dogman, I will install a Turnigy wattmeter on the bike as you suggest.