General battery questions

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?

What about them being in parallel?

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.

The motors draw around 160A each and the batteries are around $30 a piece and I cant afford to kill them.

Sorry if these are dumb questions, I don't really know much about batteries.

magruder13: Hey all, I'm using 4 12V 120Amp hour batteries to run my robot at 24V and I have a few questions.

120 amp/hour, those are big!

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?

Well one would like to have the loads balanced between the two strings, if for no other reason then maximum run time rather then having to stop to recharge one string while the other might have 30% remaining.

What about them being in parallel?

That would work OK.

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.

Just be sure the individual 12volt batteries start out with equal charge state. When charging a two battery string, it's possible to overcharge one battery of the string while the second still needs some charge. Some occational testing/measurment to ensure that the two batteries of each string are still in balance is called for. That charger seems to have a 7 amp max current rate, so plan on quite the wait to go from fully discharged to full charged for your two battery strings, could be 3-4 days in total.

The motors draw around 160A each and the batteries are around $30 a piece and I cant afford to kill them.

Only $30 each for a 12vdc 160 AH battery? That sounds awful cheap to me?

Sorry if these are dumb questions, I don't really know much about batteries.

When it comes to batteries for sure there are no dumb questions. I do hope your using properly sized fuses to protect the batteries and the robot as those batteries represent a whole lot of energy capable of starting a large fire very quickly. Be safe.

magruder13: 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.

magruder13: 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.

magruder13: 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").

magruder13: 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:

http://www.batteriesplus.com/product/35506-PS--12350J-Powersonic-Battery-35Ah-12Vwith-Lead-J-Terminal/100085-1/102626-SLA-Sealed-Lead-Acid-Batteries/102638-Power-Sonic/12V.aspx

...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?).

magruder13: 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...

???

Okay, the reason that the 120Amp Hour seems like an insane amount is because I was unsure how much it was so I looked up my battery and its 12.0 Amp Hours not 120. Sorry for that, I just read it fast and missed the "." .

Thanks for all of the advise, I don't have time to read through it all right now but later I will go through it all and see if I have any more questions.

Thanks again!

Chris

so I looked up my battery and its 12.0 Amp Hours not 120. Sorry for that, I just read it fast and missed the "." .

I suspected that :D

But what about your:

The motors draw around 160A each

That doesn't sound like a workable match at all. Or are the motors 16.0 amps draw each? ;)

Lefty

That doesn't sound like a workable match at all. Or are the motors 16.0 amps draw each?

Actually they run at 68A and stall at 190A, but I am usually driving up and down inclines and pushing other bots, so I can reach 190 pretty easily. I am using 8 gauge wire for the drive train right now.

magruder13:

That doesn't sound like a workable match at all. Or are the motors 16.0 amps draw each?

Actually they run at 68A and stall at 190A, but I am usually driving up and down inclines and pushing other bots, so I can reach 190 pretty easily. I am using 8 gauge wire for the drive train right now.

So your 12 amp/hour batteries will give you about what 10mins run time?

Lefty

Well it's a competition robot and with 4 batteries the 24Ah will keep me running way longer than I need for a 7 minute round. My team has 8 batteries total, and we have 30 minutes between rounds so we have plenty of time to charge batteries.

magruder13:
Okay, the reason that the 120Amp Hour seems like an insane amount is because I was unsure how much it was so I looked up my battery and its 12.0 Amp Hours not 120. Sorry for that, I just read it fast and missed the “.” .

Thanks for all of the advise, I don’t have time to read through it all right now but later I will go through it all and see if I have any more questions.

Thanks again!

Chris

Ah man, here I was hoping for someone doing something “crazy and insane” - but your explanation does make sense…

:slight_smile:

cr0sh:

magruder13: Okay, the reason that the 120Amp Hour seems like an insane amount is because I was unsure how much it was so I looked up my battery and its 12.0 Amp Hours not 120. Sorry for that, I just read it fast and missed the "." .

Thanks for all of the advise, I don't have time to read through it all right now but later I will go through it all and see if I have any more questions.

Thanks again!

Chris

Ah man, here I was hoping for someone doing something "crazy and insane" - but your explanation does make sense...

:)

I wouldn't mind making an arduino powered golfcart..I mean hell, they have a competition for autonomous road vehicles.