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Step 12Particulars of the Charger Circuit
Two cautions:
Because this super-simple charger has no turn-off mechanism, be very careful and be sure to pull the batteries out when they're charged. I stop charging my batteries early. They'll last longer if you under-charge slightly anyway.
And don't use it for LiIon or LiPo batteries -- they need constant voltage instead of constant current.
The LM317 chip got very hot with a few hundred milliamps current, so I built a simple heatsink by taking a copper clad scrap, drilling a hole in it, and bolting it to the LM317. Now it's pretty hot all across the copper, which means it's working -- pulling heat from the LM317. Is it enough heatsinking? Not sure without doing some math. If it burns out, I'll redesign.
Note also that the resistors are high-wattage ones. Since only 1.25v is dropped across the resistors, you should be able to get away with regular (1/4 watt) resistors for most reasonable currents, as long as you're not using single-digit resistance values. Still, mine get kinda hot. Definitely use power resistors if you've got 'em.
Most of the connectors on my battery packs are standard 2-pin female sockets as far as possible. This makes it easy to hook them up to the charger, or make an adapter you need. For instance, I took the top off an old 9v battery, soldered a wire to it, and now I can use it for rechargeable "9-volts" (they're actually 7.2v). Modular plugs/headers are your friend. I think some of my robots are 1/2 connectors by weight.
And speaking of 9v. The datasheet for the LM317 shows it needing about 2-2.5v more input than output. That's ok for me, b/c my highest-voltage battery pack is 7.2v, which wants to charge up to around 9.6v. But I'm pushing it. It might be better to use an 18v supply. Or maybe it's a good idea to slow the current down when the battery's almost fully charged?
If you want to use this type of charger yourself, go give Battery University a look. They talk a lot about good charging currents, times, discharge cycles, etc.
Because you can regulate the current very easily with this design by swapping out resistors, one set of resistors can make it a trickle-charger for charging up overnight, while a different set of resistors can charge up your batteries in 1 hour for when you're impatient.
A smarter charger is a project for another day.
Because this super-simple charger has no turn-off mechanism, be very careful and be sure to pull the batteries out when they're charged. I stop charging my batteries early. They'll last longer if you under-charge slightly anyway.
And don't use it for LiIon or LiPo batteries -- they need constant voltage instead of constant current.
The LM317 chip got very hot with a few hundred milliamps current, so I built a simple heatsink by taking a copper clad scrap, drilling a hole in it, and bolting it to the LM317. Now it's pretty hot all across the copper, which means it's working -- pulling heat from the LM317. Is it enough heatsinking? Not sure without doing some math. If it burns out, I'll redesign.
Note also that the resistors are high-wattage ones. Since only 1.25v is dropped across the resistors, you should be able to get away with regular (1/4 watt) resistors for most reasonable currents, as long as you're not using single-digit resistance values. Still, mine get kinda hot. Definitely use power resistors if you've got 'em.
Most of the connectors on my battery packs are standard 2-pin female sockets as far as possible. This makes it easy to hook them up to the charger, or make an adapter you need. For instance, I took the top off an old 9v battery, soldered a wire to it, and now I can use it for rechargeable "9-volts" (they're actually 7.2v). Modular plugs/headers are your friend. I think some of my robots are 1/2 connectors by weight.
And speaking of 9v. The datasheet for the LM317 shows it needing about 2-2.5v more input than output. That's ok for me, b/c my highest-voltage battery pack is 7.2v, which wants to charge up to around 9.6v. But I'm pushing it. It might be better to use an 18v supply. Or maybe it's a good idea to slow the current down when the battery's almost fully charged?
If you want to use this type of charger yourself, go give Battery University a look. They talk a lot about good charging currents, times, discharge cycles, etc.
Because you can regulate the current very easily with this design by swapping out resistors, one set of resistors can make it a trickle-charger for charging up overnight, while a different set of resistors can charge up your batteries in 1 hour for when you're impatient.
A smarter charger is a project for another day.
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hot = might as well not really be there because its saturated with heat
for a heat sink to really work it has to dissipate more heat than its absorbing, that way its wicking the heat away into air
try replacing that copper pcb with a flattened down "slot cover" from the back of a pc and see if you notice any difference, i bet you will :)