<from another thread>:
combining batteries in parallel is fairly simple to understand.
if you have 2 lifepo4 packs which have the same voltage then you can combine them directly without diodes.
this includes a situation in which you combine different capacity packs as well, such as combining a 10Ah pack with a 20Ah pack in parallel.
after charging them up to the same voltage with the charger, you combine them by connecting the P- leads together first, then you put a resistor across the two positive (red) leads to equalize the voltage on each pack , then connect the two red leads together. from this point on you always keep the two batteries connected and charge them up together.
do not put switches between them, do not put diodes between them. when they are connected together this way, they share the load when discharging and they will split the charging current between themselves as well so that they both will climb in voltage together and reach full charge at the same time.
if you disconnect them and one pack is discharged separately, then you have to go through the same procedure, charging up and using the resistor to finally equalize them every time you recombine them in parallel.
you should never use a lifepo4 pack wihout a BMS, combining the two packs in parallel does not affect the BMS which functions normally and when one pack has reached LVC and the output mosfets shut off on that side then the current will continue to be delivered by the other pack until it cuts out at LVC.
this will be at two different pack voltages usually so you should recharge immediately when the first pack reaches LVC. at this point the battery remaining is having to produce more than twice the current it did at the start and so you want to restrict it's discharge rate at that time.
resetting the BMS that shut off and then continuing at dramatically reduced power may allow you enuff travel distance to get the packs back to the charger.
when charging two different capacity packs in parallel, the charging current will be split between the two packs proportionally to their capacity and they will both charge up to their full capacity at the final charger voltage. and then they balance at that voltage until the charger shuts off.
you should not combine two packs in parallel with different voltages, such as a 36V and a 48V pack even with a diode to protect the lower pack from the upper pack.
but if you have different chemistries, such as nicad or nimh or SLA combined with the lifepo4 in parallel you need to use a diode on the output of two packs to combine them, to prevent current from flowing from one pack to the other.
for this situation where you combine the two different types of packs, you would select a schottky diode capable of withstanding the maximum voltage difference between the two packs.
so a 48V nicad pack fully charged is about 54V and a 48V lifepo4 pack is about 58V so you would need a schottky diode of at least 4-5V but something around 20V is the smallest they sell for these power diodes and the higher the breakdown voltage the more expensive it costs. but more important is that the forward voltage across the diode when it is conducting is directly related to the the reverse breakdown voltage. you want this forward voltage to be small because that determines how much heat the diode produces. power =VxI.
the schottky diode has three legs in the To-220AB package. the current comes out of the center leg called the cathode, and you connect each battery's positive terminal to the outside leg of the schottky. that is called the anode.
all the current from the two packs flows out of the center leg and that is what you connect to the motor controller positive lead.
you can also use two axial schottky diodes, one for each pack, with the cathodes soldered together to the controller lead and the anodes to each of the packs.
after you know what voltage diode you need, then select the diode current carrying capacity that is able to handle all the current you expect your battery pack to produce at maximum discharge. this is why they are expensive because a large schottky capable of handling a lot of current gets expensive fast.
the problem with combining different battery chemistries in parallel is the restricted voltage range of the older SLA and nicad chemistries compared to lifepo4. a fully discharged 48V nicad pack is around 40V but the 48V lifepo4 pack will continue producing current down to about 32V.
to protect the nicad pack, the controller LVC of 40V will shut off the controller so the nicad pack is not over discharged. this leaves a lot of charge on the lifepo4 pack so you only are able to produce about 65% of the lifepo4 pack power.
so if you wish to combine the lifepo4 and the nicad, use a 36V controller and a 36V nicad pack. the 30V LVC of the controller will protect the nicad pack. then you would combine a 48V lifepo4 pack with the 36V nicad in parallel using a 20-30V diode since the nicad would be charged to about 40V and the fully charged lifepo4 would be around 57V.
in this case, the nicad would be restricted from releasing current through the diode because the output voltage is so much higher than the nicad fully charged voltage, and the lifepo4 would not be pushing current inot the nicad because the 20V diode blocks it.
when the lifepo4 pack has been discharged down to the 40V of the nicad then the nicad would add current through the diode to the current from the lifepo4 until the LVC of the controller shut off. this protects the nicad. the BMS on the lifepo4 protects that pack.
in this case with the 36V nicad and the 48V lifepo4, you would only need one axial schottky diode on the output of the nicad pack, and no diode would be needed on the lifepo4 at all.
looking in mouser catalog, ON semiconductor 1N5817G, 20V 25A axial diode is $.20. it has Vforward of .45V so when the max 25A current is flowing the diode is only producing about 10 watts of heat.
but that heat is only produced at the end of the discharge of the packs. if you also had a big diode in the lifepo4 pack output, it would be producing a lot more heat because it produces more current over a longer period, so it would need a heatsink.
but that axial diode on the nicad pack will get hot so mount it so it is not touching anything that will melt and is out in the open air for cooling.
that is the only situation in which i can see the need for diodes to combine two packs in parallel. a 36V nicad and a 48V lifepo4, in which you would be able to get the maximum capacity out of the two packs. and only a 20 cent diode needed.
this requires common sense to manage. charge the nicad up first and the lifpo4 next to insure that the voltage between the 2 packs does not exceed the 20V breakdown, or choose a 30V diode to be safer, 1N5818G 20V 25A .55V forward bias, 13 watts of heat at max current.
you can also find the same features in TO-220AC packages, so you could dissipate the heat with a heat sink too.
