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Battery Charge Controller using a VRDC

  1. Nov 28, 2012 #1

    I am trying to use a DC voltage sensing relay to control a generator in order to charge my batteries when they get low on charge. I have two 12 volt batteries in parallel connected to an UPS Inverter/Battery Charger. This charger is powered by the generator. I want to connect (in parallel) a VRDC to the batteries to sense when they are discharged to a specific voltage level, in turn, starting the generator and begininng to charge the batteries.

    My question lies in turning the generator off. The VRDC has a cut out that can be set which would disable the generator at a specific voltage sensed across the batteries. I want this to be when the batteries are fully charged. But the problem with this is that chargers begin by bulk charging at say 14.4 volts and once they equalize, drop the voltage to like 13.4 volts to float the battery. A fully charged 12 volt battery should read about 12.7 volts. How do I set the VRDC to accept the higher voltage from the charger but somehow know when the the battery is fully charged? How do regular chargers manage this? If I set my VRDC to turn the generator on at say 12.2V and then off at say 14.5V (to allow for sensing the charge voltage of 14.4V), when will the generator ever turn off??

    I'm stumped, any help is appreciated. I've added an attachment of a quick diagram to visualize.


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  3. Dec 2, 2012 #2


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    Staff: Mentor

    Hi Engineerin4me, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    I see you haven't been overwhelmed with responses, and I'm afraid I'm going to be of little help, too. But you might have to resort to some form of active sensing (fully automated, of course) such as disconnecting the charger, waiting a few minutes, then briefly switching in a heavy load, say 10A. Noting how much the battery voltage drops will indicate whether the battery is in a good state of charge or sadly woebegone and needing more. Then proceed accordingly.

    The care and feeding of lead batteries is a mature technology, so I expect that some ferreting around using google will turn up all the information you need.

    Your block diagram shows a pair of batteries connected in parallel. Is this really how you are set up? If so, it can be expected to become less and less effective as the batteries age and their differences become more pronounced. In time, that arrangement may offer little more than the capacity of just a single battery, even when the batteries have initially been matched as closely as possible.

    Good luck with your electrifying project! :smile:
    Last edited by a moderator: May 6, 2017
  4. Dec 2, 2012 #3


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    Gold Member

    It's been nearly 30 years since I've properly charged a lead acid battery, so lately I've been going to Battery University for my refresher on how to answer these type questions.

    First, I'd like to echo NascentOxygen's comment about it not being a good thing to have two batteries in parallel.

    An automatic charger should follow the procedure outlined at Battery University, so you will need a current sensor at your battery to send data back to your VRDC. Which I've just discovered stands for "Voltage Sensitive Relay for DC". And looking at it, it doesn't have a current sensor input. Well then. Ignore what I said about the current sensor input. But without that, you're now stuck with simply a voltage sensor, which according to Battery Univ, will shut the generator down at only 70% charge. I don't think I like this system at all. You really need to be able to monitor current into the battery to determine it's state of charge. My god this is getting complicated. It's no wonder I've never designed one of these. I suppose you could acquire an under current sensing relay to shut down the generator. That would work.

    Lastly, discharging your battery to 12.2 volts before starting your generator will reduce the life of your battery. 12.2 volts is around 50% depth of discharge(dod). Starting the generator at 12.45 volts(~25% dod) will double the life of your battery.

    Sorry for all the links, but batteries are very complex creatures, and you should know as much about them as possible. They can be very disappointing if not properly cared for.

    ps. Lead Acid batteries explode when abused.

    pps. My links to the relays were simple google search results, and not meant to imply any affinity, nor endorsement, on my part, for the indicated products.
  5. Dec 4, 2012 #4
    I appreciate the responses, very good info. I'm not 100% sure if the batteries are in parallel or rather in series. This is a remote site, hence the need for batteries and a generator and my drawing only shows 1 battery when I know there are two. So I will have to look into that, but great info, I was unaware parallel is not good for batteries.

    I think I will look into current sensing, that was kinda my hunch too. And I will check out Battery U and try to get a little more knowledge of batteries under my belt.

    So thanks again for the greaat comments!

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