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Charging two batteries at once. — Parallax Forums

Charging two batteries at once.

DgswanerDgswaner Posts: 795
edited 2009-04-25 03:33 in General Discussion
I'm working on some ideas for a recharge station for my bot. I have 2 batteries, 1 12v deep cycle which run motors servos, etc. and a smaller 12v sealed lead acid battery that runs a few sensors and a Propeller. I have a charger which I have to connect to a battery, then the other battery once the first is done charging.

Q: can I connect these batteries in parallel, with Diodes when charging and charge both at the same time?

my other though was to toggle a relay every so often switching from one battery to the next, but this seems like hack and not a solution.

any input is welcome!

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DGSwaner
"When in doubt, use C4" - Jamie Hyneman, Myth Buster

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-04-23 18:58
    I would not try to charge these batteries in parallel since they're very different types of batteries. There are some fairly simple chargers for sealed lead-acid batteries that you could make and permanently wire into the circuit. I believe the deep cycle batteries require a specific charging profile for best performance.
  • DgswanerDgswaner Posts: 795
    edited 2009-04-23 19:42
    Thanks mike, glad I checked before I ruined something.

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    DGSwaner
    "When in doubt, use C4" - Jamie Hyneman, Myth Buster
  • Mike GreenMike Green Posts: 23,101
    edited 2009-04-23 20:41
    This regulator seems to be made for this kind of application: www.st.com/stonline/books/pdf/docs/1318.pdf
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-24 05:09
    Mike Green said...
    I would not try to charge these batteries in parallel since they're very different types of batteries. There are some fairly simple chargers for sealed lead-acid batteries that you could make and permanently wire into the circuit. I believe the deep cycle batteries require a specific charging profile for best performance.
    Actually they don't.· You can use the same charger for either, because the basic chemistry is identical.· There is no basic difference between a flooded lead-acid (FLA) cell and a sealed lead-acid (SLA) cell (of either gel-cell or absorbent glass mat variety) that is in any way analogous to the difference, say, between NiCd and NiMH cells.

    However, there are problems charging any two lead-acid cells in parallel if they aren't discharged in parallel -- especially if they are of widely disparate sizes and if they are discharged to widely disparate percentages of capacity.

    If you isolate them with diodes, you can charge'em in parallel, though.· The deadest one will charge the most, and you must compensate for the diode drop.

    Me, if I absolutely had to have two batteries, I'd just get two 12- or 15-volt remote-sensing Lambdas (one for each battery)·on eBay, where they show up all the time (I bought ten 8-amp ones for under $50 total); adjust them to 13.5 volts; use a diode with each to protect the Lambda from the battery; and remote-sense them to compensate automatically for the diode drop.· In fact I not only would do that, I have done that for years.· Works like a champ, though a float-charge setup like that won't charge a dead battery as quickly as a smart·multiple-level charger will.· I use 8-amp Lambdas (permanently installed) in my camper/motor home to float the vehicle battery and house battery separately when parked.· When the engine is running, they're charged in parallel, tied together by a relay.· You could do that too.

    But why are you using two batteries anyway?· I'd run both loads from one battery (the large one) and avoid the whole danged hassle.· You've got so much extra voltage to feed the regulators for the electronics that you could even do it in two stages (say, a 9-volt regulator feeding a 5- or 3.3-volt regulator, with filtering in between).· Either way you can get super-clean power for the electronics.· Saves weight, space, and money·too.

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    · -- Carl, nn5i@arrl.net

    Post Edited (Carl Hayes) : 4/24/2009 5:16:27 AM GMT
  • DgswanerDgswaner Posts: 795
    edited 2009-04-24 19:00
    wow, I'm going to have to read through that a couple of times...... where I have the one charger already, I'm leaning toward the L200 regulator mike suggested.

    They reason I have 2 batteries is that, I was having issues with my Propeller resetting under heavy loads (moving). My bot is about 16" in diameter, 2' tall 40lbs+. I unfortunately got some bad advice from the place I bought my battery. I did have larger motors at the time but under there recommendation I bought a deep cycle battery. it's a pretty small for a marine battery. slightly larger than a motorcycle battery but not the best option. Besides once having larger motors, I was planning on having an on board PC. and wanted 2-3 hours of run time.

    the biggest mistake about having 1 12v deep cycle battery is: I have 12v motors, so it doesn't take much for the battery to drain and the motors to start slowing. I'd prefer to have 14+v from the battery(s) regulate it down to 12V and have more consistent motor speeds. .... off to Google Lambda.

    thanks for the input.

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    DGSwaner
    "When in doubt, use C4" - Jamie Hyneman, Myth Buster
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-24 22:20
    Are they really deep-cycle batteries?· Or are they "marine" batteries, which are an intermediate sort of design?· I would select the sort of batteries used in golf carts, which are no-compromise deep-cycle designs, intended for exactly your sort of application (supplying electric motors).

    Your resets are more probably caused by spikes from switching motors on and off than by low voltage from the batteries.· Filtering can take care of that, along with kick diodes across the motors to absorb the inductive kick.· After all, you're regulating all the way down to 3.3v, while a 12v lead-acid battery that drops to 10 volts is about 95% discharged, or worse.

    My Lambdas, which serve extremely well, are model JWS100-12.· There's a vendor on eBay that sells these in sets of five, and has been doing so for at least a year.

    http://cgi.ebay.com/5-LAMBDA-JWS100-12-A-12VDC-8-5A-POWER-SUPPLY_W0QQitemZ120410315751QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item120410315751&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50

    That's where I got mine.· I guess I remembered price wrong -- I thought I had paid $50 for ten of them, not five.· Memory plays scurvy tricks.· Anyway, they were all brand new, and I'm pleased with them.

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    · -- Carl, nn5i@arrl.net

    Post Edited (Carl Hayes) : 4/24/2009 11:50:06 PM GMT
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-25 03:33
    I neglected to mention that those power supplies are adjustable by an externally available potentiomenter, from about 10v to about 14v. Very useful for many things.

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    · -- Carl, nn5i@arrl.net
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