To Dweezil: balanced charging

2 replies [Last post]
Joined: 12/01/2006
Points: 711

So now instead of a scooter, I have a project. What I'm left with is the 5th battery, 12v 18AH. What's the difference between that and 12v 22AH? Just more time between charging, i.e. longer range? Should all 5 batteries be the same for sake of balanced charging?

AH (assuming they have both been rated honestly using the same protocol) is a measure of power storage capacity. Yes, 22 is about 20% greater than 18. This would normally mean 20% more lead. However, both these batteries are the same outside dimensions. The extra capacity must come from somewhere; inside a battery there is lead, plate dividers, air, and electrolyte. I don't know what is reduced to increase the number of available electrons, aside for a configuration change where the top air space is reduced.

My goal now is the standard 60v mod that has been so much discussed here. I guess it should be straightforward... the controller shunt, all 5 batteries wired together (5th battery either under the seat or that special mod under the bike) and wired to the charging port, and can be charged on the bike in a balanced way with a new 60v charger. Is this a pretty standard mod by now?

Charging using a serial 60V source is simple. However, it vitally depends on the precise matching of the capacity between the batteries in the string. The more the differences between the batteries, the shorter the life of the pack. Chargers are also expensive.

Imagine you had five fuel tanks and filled them from five hoses, each set to meter a precisely equal amount of gas. Fill is terminated when the TOTAL weight of the set of tanks reaches a predetermined value. If each tank is the same size, there's no problem; all are filled at the same time, and the total weight trigger will stop the fill before any overflow. If one of the tanks has a slightly smaller capacity, however, it will spill over before the other batteries have reached their limit. In the case of a battery, beyond that value, water in the electrolyte is broken into hydrogen and oxygen. A very modest amount of gas can be tolerated, both by retaining the pressure and (by report) some sort of catalyst, though I have never seen such a device when I've cut cells apart. After that, the gas is vented, and the battery cooks, REDUCING FURTHER its capacity.

An alternative is the serial/parallel setup. Cables from all batteries are led out to a gang jack. In drive mode, a plug is wired to link all the batteries in a serial string, giving your full pack voltage. In charge mode, a different plug is used, wired in parallel. A 12V charger is used. If a battery reaches its capacity, its internal resistance will go up, and the electrons will be diverted to cells that still can use the power. Charge is terminated when all batteries reach their termination voltage.

It takes more wiring, but the result is worth the effort, I believe. It can accommodate a pack composed of unbalanced batteries, though they may still cause trouble on discharge, as a low capacity battery will be drained before the others, and may be drawn into dangerously low voltage levels.


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ebiker_JK's picture
Joined: 12/18/2010
Points: 83
Re: To Dweezil: balanced charging

Thanks for this, mf70. My battery pack is OLD so perhaps when I buy a new pack and can do all 5 at once which would give me a balanced pack to start with.
Currently, I'm juggling different battery types and I know it's not as effective as it could be. Likewise, the charging system is handled with two devices.

dweezil's picture
Joined: 05/24/2011
Points: 15
Re: To Dweezil: balanced charging

Thanks, mf70, for the info. I did end up buying 5 new identical batteries. It's taking me awhile to get the bike back together, I had one of the batteries out of its box for a month, sitting in the bike to figure out the 5th battery location. Yesterday I checked it with the voltmeter, it was at 11.79 volts. The four other batteries still in their boxes were all 12.54 - 12.60. So I charged up the first one, I had to take it over 13v and then it slowly dropped back down to 12.55, now I guess I have the five batteries balanced.

But now I have a new problem, I've hooked up the controller after having made the shunt mod and connected one battery so far - the 5th one under the seat. But now the two leads (black and red) running into the 4-cell battery box show the 12.55 voltage between them (coming from the 5th battery). But should there be any voltage at all if the key is not turned on? I'm afraid to connect up the four batteries in the battery box because it seems there may be a short. Maybe I screwed up while soldering the shunt mod?

Basically, the question is, should there be a connection between the black and red power leads coming from the controller if the key is not turned on? Because right now, I appear to see a voltage being passed through the controller with no key in the ignition. I know that doesn't necessarily mean amperage would flow, but still, it seems wrong, at least to a hardware novice like me.



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