Mixing different Ah size packs ?

[greasypants]

Is it a problem to mix battey packs with different Ah ratings?
Trying to decide which nimh packs to get for my bike . I will probably go with a 5304 in the rear and was planning on 72v . Was considering 2 37v 18Ah packs (23 lbs each), mount one in the frame and one on the rear . But I think these would be abit to wide to put in the frame triangle after I build a box for it , might be uncomfortable when riding . Now I am thinking of a 36V 14 Ah (18 Lbs ) pack in the frame ( 1 inch narrower ) , and 2 24v 18Ah (15.5 Lbs ) packs mounted on the side of a rear rack in custom build waterproof boxes . This would keep the rear weight a bit lower and would also give me 84 V at 14 Ah and 48v at 4 Ah left over. Now what happens when the 36v pack is drained at 14Ah , will the voltage drop to 48V and continue putting out amps until the other packs are dead ? Is this a bad thing ? Can the 36v pack be damaged ? I thought I would run my lights of the 24v packs to use up some of the extra Ah. My round trip commute is about 33 km and I would like to be able to ride pretty hard with out running out of power .

 

[xyster]

Now what happens when the 36v pack is drained at 14Ah , will the voltage drop to 48V and continue putting out amps until the other packs are dead ?

Yes, and the polarity in the 14ah pack that's almost fully drained will begin to reverse, probably ruining them permanently.

I thought I would run my lights of the 24v packs to use up some of the extra Ah.

Running the lights off the 18ah packs is an excellent idea. If I was thinking of doing this, I'd get one of justin's drainbrain's installing it so as to measure the voltage and drain of the 36v pack only. Then cut the ride when it reads about 31-32 volts. Or, if you can test that 36v pack's capacity alone, then when in series with the other packs, ride only until the 36v pack is ~80% completely drained.
One problem with NiMH can be widely varying capacity in the same pack from day to day. Aerowhatt at the old V (our previous forum) reported his NiMH pack would some days put out 13ah, some days 9ah, for no apparent reason. That was one reason he said he ditched the NiMH for lithium -- he could never be sure exactly how far he could ride, and because his was a heavy scooter, he couldn't pedal it or easily walk it home.
In your case, any variance could prove deadly to the smaller pack, so I'd definitely be watching it's voltage like a hawk, and not just it's % capacity drained.

 

[xyster]

Another possible cheap solution would be to build a low voltage cutoff for the 36v pack with a relay or relays of appropriate voltage, that are engaged by the power of the 36v pack. You'd have to test the relays carefully, trying to get one or more in series, or in series with a resistor, to reliably disengage at 30-32 volts.

Probably other folks here can offer better LVC solutions using MOSFETs or transistors.

 

[greasypants]

I thought about putting in a switch to shut off the 36v pack manually , I suppose I could just use a volt meter and turn it of when it hit a certain voltage .

 

[Matt Gruber]

u should build a mock-up of the wide pack and ride around.

keep the packs equal, unless they are free or low cost.

 

[xyster]

I thought about putting in a switch to shut off the 36v pack manually , I suppose I could just use a volt meter and turn it of when it hit a certain voltage .

U could wire a SPDT switch to switch-out the 36v pack when you see it's voltage is low. This way, you can keep riding at 48 volts until the two 24v packs are drained. P'rolly how I'd do it.

 

[greasypants]

Matt I took your suggestion and build a box out of wood the same size as the 36V 18 Ah pack and duct taped it to the frame . Rode around for awhile with no problems. So I think I will get 2 37V 18AH packs and run 72V . This way I can mount the 2nd pack on a rear rack and still have room for panniers to carry all my crap for work . Can add more later if I feel the need.
Thanks
Greg