Amp hour question.....

I have found a dc to dc converter to step down the 58vlts from my ebike li-ion pack to 12vlts.
If the pack is rated at 12ah (at 58vlts) when stepped down to 12vlts it should be 58ah, correct?


I figure what actually is useful is Watt hours, this pack would be rated at 696 watt hours, when divided by 12vlts it comes out to 58ah.....

Reason I am doing this is to power my CPAP while on a 4 day sailing trip. Manufacturer of the CPAP recommends the use of a battery rated at 8ah for one night, so if my understanding is correct I’ll have more than twice what I need for three nights....

This is assuming that the 95% efficiency of the dc-dc converter is good. It’s made by Grinn, the guys that make the satiator charger so I’m betting it’s good.

In theory, yes.

In reality, the advertised efficiency is the peak efficiency. The efficiency graph isn't usually flat usually dropping off more, the bigger the change in voltage.

I'd factor in more like 85-90% efficient.

Your math is right. 58v 12ah converts to 12v 58ah.

I'm not familiar with that converter. If Justin/Grin says it's 95% I'd trust that, but there may be caveats to that. DC-DC converters rarely have a set efficiency that's the same over all voltages and loads. Some are rated for a specific conversion and load, but might be less efficient at lighter loads or larger voltage changes.

For example, I had generic ebay DC-DC converter that was rated at 90% efficiency at 1 amp. and it was.... What they didn't say was that it was 0% efficient at .1 amp, as it would draw a constant .1 amp at all times. Since your health relies on this, it would be best to test it under load and make sure.

Thanks for the responses
The converter should be here on Monday, maybe sooner. The dc “power supply” from the CPAP manufacturer will be here on Thursday next, giving me a week before departure to give it a run.

I think it’s pretty cool to be able to use a big ebike battery for many things, always kinda bugged me to have 600watt hours on my bike and not be able to charge my phone....

The manufacturer has their own dc -dc converter, I’m guessing that it’s purpose is to even out fluctuating input voltage, it also has a fuse holder.
Under normal AC use it has the usual “brick on a rope” AC - DC supply that outputs 12vlt DC

With a quality dc -dc converter like the one from Grinn I’m thinking the “stock” converter would be redundant. But this unit is far from cheap so I’ll use it and hope that it’s load, or inefficiency is minimal with the input already being a good even 12vlt...

Do you think it would be risky to plug the Grinn unit straight into a laptop?

BTW I have lived for years without the CPAP so I think a night or two without if this setup fails won’t be so bad. However I’m told that my snoring is so bad I’ll wake up in the dinghy if I don’t use it.....