charging one battery from another

i have built a small portable power pack (4s6p 18650 with a 10A BMS charger for each 4s2p block) with about 180Wh for a camping trip. build log will follow. i want to attach 2 power tool batteries to the pack (Einhell Power X Change 5ah 18V) too add additional capacity. i want to be able to charge the pack from the power tool batteries i plan to use the external batteries in parallel but I want to be able to put in batteries of different charge level and capacity eg a full 5ah and a empty 1 ah side by side without over charging the 1A.

the pack runs of a 14.8v nominal bus with dc dc converters branching of for different outputs (regulated 5v 6.4v and 12v) mode selection is done by a 3 way switch from the battery pos 1 connects the battery to the charging port (24V input) pos 2 is the off position and pos 3 connects the battery to the main power bus = on.

would it be possible to charge the external batteries too by adding some more dc dc converter backwards. with the same selector switch for each external battery as the main battery has


i do not know what the einhell batteries need as a charger but i will investigate further.

for reference Einhell = ozito in GB and australia
best regards Albach

its possible but sounds like you are complicating it a bit. More dc dc converters= more losses.

Or are you carrying your tool bats to work and to charge from the other? Then its understandable

Thanks for the post.

So you could do some of these combinations:
Einhell tool bat > DC stepdown > 4s6p 18650 pack
One Einhell tool bat > one DC stepdown > loads directly
4s6p 18650 pack > one DC stepup > one Einhell tool bat (one at a time)
4s6p 18650 pack > two separate DC stepups > two separate Einhell tool bats at different charge levels

If the Einhells were different charge levels, just connecting together would be a bad idea (high currents would flow between them until they equalised)
If they were equal charge level, then you could connect together & charge with one up stepup + discharge into 18650 pack with one stepdown.

To charge the Einhell tool bats (18V) from the nom 14V 4s 18650 pack you'd need a stepup with adjustable current limiting &adjustable output voltage to provide safe CC>CV charging

Most important when running packs like this is that you can shut off all dc converters that you can when not in use.
Every inverter will have an idle load that wastes energy

Btw can you draw up on how you intend to connect the different batteries with arrows showing what way the power should flow?

Firstly i want to excuse my long absense. I was sick for a few weeks but its getting better. Back to work.

I drew a corse schematic it is not perfect and maybe i have overloocked some of the details.




thank you for your replies and help

Just some general advise:

1. Try to use as little converters as possible! Each will complicate it and it gets less efficient.
2. Buck-converters are generally more efficient than boost-converters, so try to avoid the second by having enough output voltage from the main battery pack. That means design your pack that way, that you can do without boost-converters (as many cells in series as nessecary)
3. Don't overcomplicate things. As little connection ports and parts as possible.
4. Make sure you can switch off any converter, as already mentioned stand-by current will drain the pack over time significantly.