Hey all, I'm using 4 12V 120Amp hour batteries to run my robot at 24V and I have a few questions.
Since I am going to use them as 2 sets of 2 batteries in series, do I need to worry about how the load is distributed between the batteries?
You probably don't want to distribute the load in such a way that one set will run down faster than the other set in parallel - this could lead to issues (fire is a possibility). It would probably be best to just have a 24 volt "main bus", and then use regulation (switch mode would be the most efficient) from that bus to drop down to the voltages you need for the rest of the system.
What about them being in parallel?
There shouldn't be an issue with have two sets of 12 volt batteries in series, paralleled together (2 x 12 parallel to 2 x 12), as long as you don't let one set run down quicker than the other set. If you allow that to happen, one set could start trying to charge the other, which will lead to heat buildup, and could potentially ruin a battery or set. Set them up as one large battery, with a single voltage output bus.
You'll also want to use heavy gauge wire (or copper bus bars) to connect the batteries together. another good idea is to install fuses between the series connected batteries, as well as between the parallel sets. Finally, add a fuse inline to the main bus as close to the battery as possible. Make sure all the terminals and bus-bars are covered to prevent any short-circuits from possibly happening.
A friend said that I can charge them in series because our charger support it. This it the charger I have http://www.robotmarketplace.com/products/0-GPMM3153.html.
That charger should be OK - it says it can charge Pb batteries (whether this mean SLA, or wet-cell batteries - you might want to make sure of this - SLAs are different from a wet-cell lead-acid battery, sometimes one charger will support both types, other chargers only support one or the other) - and it says it will charge a 24 volt system. The only downside is that it only output 7A of charging current; this will be well under your pack's C/10 (slow charge) rate - your pack will have a 240 Ah capacity - your charger should really be a 24A charger (which would about 12-14 hours to charge the pack); at 7A, it will take 3-4 times as long to fully charge the pack (from "dead").
The motors draw around 160A each and the batteries are around $30 a piece and I cant afford to kill them.
You gotta be pulling my leg! Where did you find 120Ah 12 volt SLAs for $30.00 each?
This battery is the PowerSonic PS-12350J:
...its only 35Ah and costs about $75.00-80.00 each...
From what I'm seeing, 120 Ah AGM deep-cycle batteries run about $300 to $400.00 new - not only that, but they seem to weigh in excess of 100 pounds each. Are you certain of those specs? Something doesn't seem right to me here (just how big is this robot, anyway? I mean, the batteries alone would weigh over 500 lbs total. I'd also like to know what motors you're using?).
Sorry if these are dumb questions, I don't really know much about batteries.
If you "don't know much about batteries", and these really are 120 Ah 12 volt batteries - then you are most assuredly "playing with fire". This is a pack worthy of running an electric vehicle (hey, for all I know, that's exactly what you're building - except its self-driving). This is a pack that, should there be a short, will cause all hell to break loose (I can just see a wrench vaporising here - shortly before the hydrogen explosion spraying battery acid and plastic shrapnel everywhere).
Either something isn't adding up (costs, weight, etc) - or as a newbie, you are taking on a project that, should you mess up, might cost you dearly...
So which is it? I'm not saying that such a large scale robot isn't possible - I just can't believe the batteries of that rating only costing $30.00 (unless they're used - in which case I wouldn't trust the Ah rating on the batteries at all) - it's shades of that earlier $20.00 10 inch touchscreen LCD (IOW - too good to be true). Furthermore, I can't imagine somebody being a newbie to batteries on this scale, yet attempting to use them to build a robot that would have to be almost as large (or at least weigh as much) as a full-size automobile (with motors capable of moving such a mass). It doesn't seem impossible - but it does seem improbable - or foolhardy at best...