Rheostat and adding another battery to ebike for more volts

[Dougt]

YOu won't get anything at all out of adding a single extra battery in parallel with one of your existing batteries, other than a heavier bike and a lighter wallet.

I don't think that is right. Adding another battery of equal capacity and voltage (in this case 48V 10ah SLA) in parallel doubles the Amp-hr capacity and current capability. There is no need to tie the individual cells together, as this is SLA chemistry and I doubt very much his bike has any balancing through a BMS or charger. Even if it was lithium batteries this would be true , but very very risky, as the BMS would not be able to detect a fault in the second battery and the charger could overcharge a cell in the second battery - but the range and current capability would be increased. SLA is very tolerant to over-charging, over-discharging, and other abuse.

He will gain mostly doubled range (assuming, adding an equal battery to what he already has) but he will also gain a little power in that his battery will be less depleted (ie higher voltage) during equivalent riding distance vs having half the battery capacity.

 

[Eclectic]

I think I understand what you are trying to do. It seems like you want to create a battery pack that has too much voltage and then constantly adjust that voltage using a rheostat so that it does not blow up your controller. The problem with a rheostat is that it uses resistive wire to make adjustments. Resistive wire creates a lot of waste heat. With something like an audio preamplifier, the current is so low that the little bit of extra heat doesn’t matter so using a rheostat for that is ok. Unfortunately your application is very high current so the amount of wasted heat energy would make using a rheostat impractical for your application. Controlling current electronically is pretty much what your controller does. Trying to use your plan of adding extra voltage and controlling it electronically would mean you would need a controller in front to protect your controller. Not very practical.

Since the rest of your bike is 48V, it is not very practical to use a higher voltage controller.

That pretty much leaves one option, which is to increase the number of Ah not the voltage.

You could add more Ah of 48V SLA (12Ah@48V + 5AH@48V = 17Ah@48V). This would give you more distance and the voltage would not sag as fast but it is heavy and takes up space.

Another option is to change to a different type of battery chemistry. A battery that would have the same voltage but holds a lot more Ah in a smaller, lighter package. The battery that Dogman Dan recommended would be a very safe and practical solution (it would probably double your range and give you slightly better speed).

I'd look into a 48v 20 ah lifepo4 (or other lithiuim) battery for it, like a pingbattery.

 

[amberwolf]

YOu won't get anything at all out of adding a single extra battery in parallel with one of your existing batteries, other than a heavier bike and a lighter wallet.

I don't think that is right.

It *is* right, becuase he is asking about adding a *single* 12V SLA in parallel with just one of the existing 12V SLA in a series string of them. See the quote below:
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=66431#p998865

not enough room on the bike for a full 48 volt battery system, the bike is 140lbs as it is now, maybe adding parallel one 12 volt would get me the distance i want

Without *also* adding yet another 12V SLA in parallel with every other 12V SLA in that series string, none of the capacity of the single extra one can be used.

If it were in *series* with all the other SLA, then at least it's Wh could be used, assuming that all the electronics on the vehicle can handle the extra voltage.

Adding another battery of equal capacity and voltage (in this case 48V 10ah SLA) in parallel doubles the Amp-hr capacity and current capability.

That is correct. But as quoted above, that's not what he was going to do.

He will gain mostly doubled range (assuming, adding an equal battery to what he already has) but he will also gain a little power in that his battery will be less depleted (ie higher voltage) during equivalent riding distance vs having half the battery capacity.

Only if he adds an entire second pack--which is not what he was talking about in the quote I replied to.