Hot Toddy said:
So, in summary, it should be fine to charge both at the time from the same charger provided I have a suitable diode across each battery pack charge connection to prevent the risk of any “back charging” from higher pack voltage to the lower voltage pack.
Not "across" the pack, which implies placing one lead of the diode on the pack positive and the other on the pack negative.
It needs to go between one lead of the pack and the same lead of the charger, in the polarity that prevents current flow out of the pack.
So just need to add a couple of diodes to my charge cable that can handle >2A at 60V.
The diode must be able to handle *at minimum* the highest possible voltage difference between the most empty either pack will ever be, and the most full either pack will ever be. So 60v is not really needed, but won't hurt.
The diode must be able to handle *at minimum* the highest possible current that will flow between the charger and that particular pack. If it is a 10A charger, then the diode has to be able to handle 10A, minimum. (unless you choose to lower the current limit of the charger, or the charger doesnt' actually ever put out 10A even when only one pack is connected / charging and is at it's very lowest voltage).
The diode must *also* be able to handle the worst-case power dissipation load (watts), which is the highest voltage drop the diode will create between the battery and the charger, multiplied by the highest possible current that can ever flow (which will happen at that voltage difference).
Let's say the diode drops 1V at the highest current, and that highest possible charge current (assuming you don't change anything) is 10A, so 1v * 10A = 10W. So you'd need a diode that can handle, at minimum, 10 watts of power dissipation. If the voltage drop is less, then you need less power dissipation. If the voltage drop is more, then you need greater.
If you get diodes in the TO220 or similar package, they're easy to mount to heatsinks, which will improve their ability to dissipate the heat, if necessary.
Some options are in this list (I restricted via various options down to 6, but you can change the filters used)
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Schottky-Diodes-Rectifiers/_/N-ax1mj?P=1yuoc70Z1yuoc6nZ1z0y481Z1z0z63xZ1z0y4ci
whcih is just the single-diode-in-a-package version, which you can use on either the positive or the negative wires.
Then this is the dual-diode-per-package version, so if you have lower power requirements you can use just one device with two diodes in it to do both batteries; they are dual-common-cathode so you have to use this with teh common pin on the charger negative, and the battery negatives go separately to the outside legs.
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Schottky-Diodes-Rectifiers/_/N-ax1mj?P=1z0y481Z1z0z63xZ1yuoc70Z1yuoc6nZ1z0y4bt
If you're using lower *total* charger current limit (say, 4A with the intent that this be "split" between two packs), then you can use diodes that only need 4A and 4w capability (assuming 1v drop).
A final consideration is your charger total voltage: With the diode in place, it may require some adjustment. The voltage drop across a diode drops as the current drops, but it doesn't reach zero, so you may need to experimentally adjust teh charger voltage to end up with the desired actual full battery voltage.