i have a robot, it has 3 12v batteries, the whole setup uses the frame as ground points. 1 12v battery is standalone, the other 2 are in series at 24v, i need to tap into the 24v setup to get a 2nd 12v line to the robot.

how would i tap into and 24v series, and get 12v from it?

 

That would depend upon how the two in series are grounded to the frame.

Are they grounded at the crossover strap, so that you have +12 and -12 with a ground, or is one end of the two-battery series string grounded?

If one end of the two-battery series string is grounded, then tap in where the crossover strap is.

If you're in doubt, use a voltmeter using the frame as your ground to determine where to connect.

 

the ground of the 24v series is to the frame

i just found this link
http://bus.getdave.com/Docs/12Von24V/
pretty interesting stuff

however i think im just going to use the 24v series setup just for my motors, and the 3rd 12v battery for the lighted buttons relays computer and motor controller

i don't want to complicate things with 2 ground rail systems and shortening the life of one battery overtime from unequal use.

 

also just found this, sorry for double posting

http://forums.lr4x4.com/index.php?showtopic=27665&view=findpost&p=275569

i'm doing just that, im only using the tap for relays and lighted switches, and when i charge the robot i have a 3pdt switch that hooks the batteries in parallel for charging

 

when i charge the robot i have a 3pdt switch that hooks the batteries in parallel for charging

I hope you're using Schottky diodes (one per battery) to keep them from attempting to charge/discharge each other - otherwise, when you connect them all in parallel, you could have some rather high currents flowing that could easily damage your batteries or the wiring.

Also, if/when one battery develops a shorted cell, it will cause the other two batteries to become discharged, shortening their lives.

If you use one diode per battery, it will prevent them from cross-charging. It will also cause the battery lowest in charge to receive the most charging current.

If your batteries are gel-cell, you have to be quite careful about how much current you charge them with, and at what point you terminate the charging. Overcharging gel-cell batteries will cause bubbles to form in the gel, which will permanently reduce their AH capacity.

If they're SLA (sealed lead-acid), a "rule of thumb" is to charge them at 1/8 their AH capacity for long life. But to do that, you really need to charge each cell independently.

Leaving your batteries discharged for any period of time will cause sulphation to build up on the plates, which decreases AH capacity.

 

yup, i already have the diodes in line for each sla battery i knew i need them but i dont know exactly whcih diodes would be best, 1n4001?

i charge all my batteries with the batteryminder
http://www.batteryminders.com/batterycharger/product.php?productid=16134&cat=0&page=1&featured
it charges then when the battery is full it kicks in to maintenance mode and desulfates the battery.

 

i got the whole system up and running, except for the charging setup, i bought a 3pdt switch from mouser.com and im trying to figure out what diodes would suffice.

cheers
seesoe

 

Standard rectifier diodes won't work very well with a charger like that. For one thing, with 1A flowing through them a 1N4001 diode will have a Vf (forward voltage) of around 1.1v. It will also interfere with the anti-sulphation feature of the BatteryMinder.

Schottky diodes have a much lower Vf than standard silicon rectifier diodes, but even then you'll have a problem with the "maintenance mode" not being able to function properly.

You really need more of a "round robin" approach; connecting the charger up to one battery at a time, rather than all three of them in parallel.