# What's the difference in loading outcome with a 42 vs 48 V charger? [closed]

I have a Lithium-Ion battery for an ebike. The battery is 48V, and is rated 15 Ah, so that means 15V*15Ah = 720Wh, and as it says on the battery as well.

Now it has a 48V charger (rated 2 amps). Unfortunately, I forgot that charger in the office (2.5 hours away by bike). Now I have another charger at home, but this one is rated 42V for 2 amps.

Due to the low voltage-difference, I figured it is more or less safe to connect this 42V-charger to the 48V battery.
Now assuming neither the charger nor the battery goes up in smoke if I do that, am I right to assume that it will only charge up to 42/48 of 720 Wh ? (=630Wh)

(i'm trying to procure a 48V charger, but just in case)

• I would expect no charging to take place Commented Mar 15 at 23:37
• I figured it is more or less safe.... No, it's less safe, not safe in fact. I would advise not doing it. Commented Mar 15 at 23:37
• @Colin: Yea, I know 42 Volt is less than 48V, so the ampere surges. Commented Mar 15 at 23:46
• @sotola: Possible, but why ? Is it like you can't get lower-pressure air in a high-pressure tire ? Commented Mar 15 at 23:48
• We don't know if what you call chargers are really chargers or just power supplies. We don't know what batteries you have. If you have a 48V 13S Li-Ion battery then 48V is the nominal voltage you use for calculations, but it needs to be charged to 54.6V to be full. We cannot predict the outcome of connecting something that reads 42V in place of someting that reads 48V. Commented Mar 15 at 23:48

"48V" is not enough to know exactly how many cells your pack has, but it is likely 13. It could also be 12 depending on how sleazy the marketing people are.

The 42V charger could be 10S or 11S? Check the output voltages.

In any case, here's the math that you need to understand:

This chart shows the relationship between voltage and capacity for a single cell. Note that it is highly non-linear.

Source

So if you charge a 13S battery with a 10S charger, how much capacity will you have?

A 10S charger will charge the battery to a max voltage of 42V.

For a 13S battery, that equates to 3.2V per cell.

Referring to the chart above, that will give you somewhere around 7% of the rated capacity.

Also note that you don't want to go all the way to the end of the curve as it may damage the battery. So 3.2V is effectively "dead".

TLDR; The 42V charger cannot effectively charge a 48V pack. I doubt it would be dangerous to try, but you would certainly be taking your safety into your own hands by doing so. Lithium ion batteries store a lot of energy and they can start fires if you mistreat them.