Battery - 39V vs 42V

[Black Dog]

Hoping somebody with battery knowledge can help me here. I have a Wispa 905 Classic with 16Ah battery. I am using the bike to commute to work (alternate days until I get fitter), which is a hilly route of 13.5 miles each way - so 27 miles between charges. So far, the bike has done this without a problem. A few days ago, the battery suddenly died on me totally, about 2 miles from home, leaving me to pedal a VERY heavy bike the last stretch. I was a bit concerned about this (bike is less than a year old and has done just over 500 miles), but when I went to plug it in to recharge I saw that the charger had been disturbed and the charging voltage altered. There are two settings - 39V for a conditioning charge when the bike is not in use, and 42V for a proper charge. The last time I had recharged the bike it was at 39V, not 42V. My question is - could this make such a big difference to battery life? The difference is between normally being able to cover 27 miles with power to spare, and 25 miles with the battery dead on its feet - not even a squeak left.

I am at work at the moment, having given the battery a good 12 hours' charge at 42V yesterday, and I am keeping my fingers crossed that it will get me home. If it fails, it's going in a hedge and I am thumbing a lift, because I ain't pushing it up that hill again! I recognise that the mistake was mine (didn't notice the voltage switch had been moved), but wondering if such a small difference in charging voltage (about 7% lower) could cause such a big difference in battery range. Anyone any ideas?

 

[awol]

At 39v the battery will be at about 50% capacity which is a good setting if you are going to leave it for a couple of weeks or more unused. At 42v the battery will be at full charge.
Charge up to 42v when you are using it.

 

[Deleted member 4366]

In theory, your battery will last longer if, when you don't use it for a long time, you reduce the voltage a bit from its maximum. That means don't store it at maximum voltage for months at a time. You shouldn't use the reduced voltage for normal charging and use, otherwise your battery will go out of balance because, in most batteries, the balancing happens only at the maximum voltage.

 

[Black Dog]

At 39v the battery will be at about 50% capacity which is a good setting if you are going to leave it for a couple of weeks or more unused. At 42v the battery will be at full charge.
Charge up to 42v when you are using it.

As said above, I know to use it on 42V thanks - the 39V volts was an error when I didn't notice the switch had been disturbed. Only 50% capacity? That explains it exactly. It would imply a total range on a full charge of ~50 miles, which is pretty much what I was told to expect. It got me home yesterday with no problem at all, and with 3 out of 4 'battery state' lights still lit when I got there. That was after a 12-hour charge at 42V the previous day.

Thanks for that information. Could you educate me? Why does a 7% reduction in charging voltage result in a 50% reduction in capacity? I have never understood batteries very well.

 

[Black Dog]

In theory, your battery will last longer if, when you don't use it for a long time, you reduce the voltage a bit from its maximum. That means don't store it at maximum voltage for months at a time. You shouldn't use the reduced voltage for normal charging and use, otherwise your battery will go out of balance because, in most batteries, the balancing happens only at the maximum voltage.

Very useful to know. Thank you.

 

[awol]

Thanks for that information. Could you educate me? Why does a 7% reduction in charging voltage result in a 50% reduction in capacity? I have never understood batteries very well.

Everything I've learned about batteries/cells is off this forum so there is others on here who know far more than me so anyone please correct me if I'm wrong, but as far as I know your 36v battery will have a usable range from 35v to 42v. Below 35v will still provide some power but not alot and it will drop off quickly from there. The battery BMS will cut it off at about 30v to protect the cells from over discharging.

 

[Deleted member 4366]

Here's a simulation based on actual battery discharge data. The graph shows the voltage against time in minutes at a discharge rate of 10A. The initial drop in the first minute is the 3V sag due to the 10A.

The energy you get from the battery is proportional to the total enclosed area by the red and blue lines. You can see if you charge to 3V less, you'll get about 90% of the power due to the lower voltage and 70% running time, so that makes about 63% of what you get when you charge to 42v.

 

[Black Dog]

Superb explanation. I even understand it! Thank you

(Sorry for delay in replying - been away on holiday.)