Our team wants to build a 2-wheeled robot platform. We want to use three 12v 7Ah lead acid batteries in parallel to power a 6-24v motor driver and arduino and other electronics. The current draw needed for the motors will be 2.4~4.4A(with and without payload) under normal use. Under heavy use, current draw needed for motors will be 10~19A. We are planning to use power diodes to avoid the self-charging of lead acid batteries when they put in parallel.
Since I know little about lead acid batteries and electronics, I don't know what kind of diodes I would need and also I am not sure if this will work. Do you know what else I need to consider when putting 3 lead acid batteries in parallel?
It would be a lot simpler to just disconnect the batteries when not in use. Discharge between the batteries is really only a consideration over longer periods or when the batteries are left sitting without a battery charger connected.
Or you could use 36volt motors and put the batteries in series. AH value remains the same at 7AH but current drawn is only one third (for the same motor power) so duration is increased by a factor of three.
Do you know which component can I use to monitor the power of each battery so that the arduino can have a sense of the state of power and maybe break the branch of the battery when it is fully discharged using relays? Do you think this is a good idea to protect the battery from being overly used or you have other recommendations?
My grey cells were clearly in a spin when I made my comment about increased duration.
What I was actually trying to convey was that system current would be reduced by a factor of three.
By placing batteries in parallel, the OP was endeavouring to increase battery capacity to 21AH at 12 volts, so total power available would be 252WH and he would be assuming that current load would be evenly distributed across all three batteries.
For example, say motor rating was 200W, then system current would be 200/12 = 16.67 amps. Ignoring any loss of capacity due to discharge rate, the potential duration would be 1.26hours
However, if the motors were in series the battery system would be 36 volts at 7AH. Again total power available would be 252WH. However with the 200W motor, the load current would now be 5.56amps. Duration would remain at 1.26 hours
There are benefits to be gained by having batteries in series (increased voltage)
a) the same discharge current flows through all three batteries so each gets discharged evenly
b) the same charge current flows through all three batteries so each gets an improved chance of being charged evenly
c) by reducing system current by a factor of 3, the I2R losses would be reduced by a factor of 9 so the system is electrically more efficient.
jackrae:
a) the same discharge current flows through all three batteries so each gets discharged evenly
b) the same charge current flows through all three batteries so each gets an improved chance of being charged evenly
These statements are not correct, or not more correct than if the batteries were in parallel. Batteries in series will still discharge into each other just as batteries in parallel (the original issue) and with respect to charging/discharging, you only have to look at LiPo batteries to see it's possible for them to not evenly charge/discharge.
In series, you also add the issue that if one cell is bad it (either high resistance or just plain junk) it will affect the whole string.
Just connect the batteries together with suitable wire and connectors for the max current. Use the diagonally opposite terminals to connect the external load and you will get equal usage of all batteries.
B1 B2 B3
-------- + ---------- + ----------> to external load
