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Anyone know how I can charge my battery from another larger battery?

Old 02-12-18, 08:30 PM
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Anyone know how I can charge my battery from another larger battery?

Iíve had a computer running 24/7 for a number of years off of a large lithium battery and a solar panel. Itís worked well, but I no longer need the computer and now Iíd like to repurpose the battery and solar panel. It charges during the day when Iím at work, and Iíd like to see if I can charge my 13.5 Ah ebike battery in the evening from the solar-charged battery.

Does anyone know if there is a DC-to-DC charger out there? The large lithium battery is 12-14 volts (IIRC). Iíd have to step up the voltage since my ebike battery is 52 volts.
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Old 02-12-18, 09:27 PM
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I always just run a ac inverter off of the battery, since they are so cheap. Then your current charger will work.
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Old 02-13-18, 02:13 PM
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It is typically more efficient to convert DC to DC directly rather than inverting ... but finding a plug ready unit may be difficult (how good are your DIY electronics skills?)
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Old 02-13-18, 03:05 PM
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Lithium batteries like to be charged from a constant current supply and they want it to shut down at the proper rest voltage as the current decreases. For my 13S battery, that would be 54.6 volts.

You might get your DC-DC converter to output the right voltage, but can you have it shut down the current to prevent overcharge? I think Unterhausen's advice for the regular lithium charger on an inverter is smart. A loss in efficiency isn't a big deal here. Perhaps it would be in a situation where every electron had to be hoarded.

The charger will probably run on DC if it's high enough. A 12-72 DC-DC converter might suffice instead of an inverter.
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Old 02-13-18, 04:15 PM
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Does anyone know the amount of current a standard charger draws? I'll have to read up on this before I plug it into an inverter and fry the inverter. However, the good news is that I'm not really worried about efficiency. After all, it's being charged up by the sun! A little loss here and there won't make much of a difference to me.
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Old 02-14-18, 11:28 AM
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Most of the ebike battery chargers are 2 or 3 amps. I agree your current charger hooked to an inverter would be the safest way to recharge.

----Pretty cool idea to use a solar panel to recharge a static battery and then recharge your bike.
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Old 02-18-18, 09:29 AM
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do ebike batteries typically have balancing circuitry built in, or is that in the charger? In any event, a dc-dc inverter really involves going to AC. I don't think going to 120vac really introduces enough inefficiency to worry about, particularly when the power is coming from solar. Relatively high power dc-dc converters aren't cheap, particularly for voltages that aren't in very common usage, and ebike battery voltages aren't common at all. DC-DC converter design is still a relatively difficult design task, you don't see that many diy projects involving converters.

I think what might be possible for someone with electronic sophistication is to inject DC after the input circuitry/rectification circuitry of the charger. You would need a boost converter to raise the voltage to some level above the voltage of the battery you want to charge. It occurs to me that the circuitry to do this is included in an ac inverter.
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Old 02-18-18, 10:26 AM
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Originally Posted by unterhausen View Post
do ebike batteries typically have balancing circuitry built in, or is that in the charger?
Yes, one of the tasks of the BMS in the battery pack is to manage the balancing.
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