36 Volt Battery Charger

Ok, Im new here but I think this is the right area to post in.
I am looking to charge a group of batteries that are 36volts(together). I have looked around but have not seen anything that explicitly states whether to use a panel/panels that out put the same amount of voltage(ie 36V) or whether I should use a panel that has an increased voltage. I am of course worried about having enough voltage to charge the batteries. So the question is, do I need to have a panel that outputs more voltage then the batteries take? or is 36 volts fine, as I have seen that panels will usually output more then their stated rating although I can not find anything to back this up or rufute it; other then voltage changes with temperature.

Also, once that question is answered, what controller can be recommended that I use? The amperage going to the batteries is no more then 10 amps as this is mainly to help keep them topped off and space is somewhat an issue.

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 36 Volt Battery Charger

    chris,
    you will find that for a given battery voltage that a much higher voltage is needed to charge the battery. generally a 12v battery will need in excess of 17v from a pv (solar electric panel in layman's terms but is short for photovoltaic) to do this. when the battery is 24v the charge voltage from the pv doubles so it's greater than 34v. there are pvs that are 35 to 36v and that is not high enough to charge the 36v battery bank you have as the pv voltage will be about 51v and higher (roughly 3 12v pvs).
    the sticky point is that this 36v is an odd output voltage that the more expensive controllers can accommodate. many of these are known as mppt and can take even higher pv input voltages like that of not just 3 12v pvs but like 4 12v pvs or 2 24v pvs or even higher voltage combinations. you should stick generally to the same pv when making up an array as mixing and matching can get tricky. not all mppt controllers will do the job so you have some research to do.
    if the guys are aware of a cheaper pwm controller that works with 36v outputs then chime in with that option for chris, but i couldn't think of one of hand.
  • System2
    System2 Posts: 6,290 admin
    Re: 36 Volt Battery Charger

    Thanks for the reply. So your saying that I would want a voltage of up to 40% higher then the actual battery voltage. That seems quite high. However just sticking with the 12 volt increments is the easiest thing to do, so 48 volts it is.
    Just a question though, I assume that as you lower the voltage it would simply hurt efficiancy until of course there isnt enough voltage to charge with?

    I have looked at different controllers and from what I see, not many are rated specifically for 36 volts. I do see however that a brand like Xantrex is in the $500 area. While certainly not as cheap as I would like, it is likely something that would only be considered after I am unable to find something that is suitable. I will of course be looking around but any help is appreciated.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 36 Volt Battery Charger

    Part of the problem is that solar panels are temperature sensitive... A "12 volt panel" with Vmp=17.7 volts may have Vmp=14 volts on a 95 degree day and assuming a 63F (35C) rise in cell temperature over ambient (little wind, not good panel ventilation).

    So, your 3x17.7 volts becomes an array Vmp=53.1 volts for a "36 volt" battery bank and on hot days it may only output 42 volts (or a bit more) to charge your bank.

    The Morning Star Tri-Star 45/60 amp MPPT controllers do support 36 volt banks ($412 for the 45 amp version).

    What is the 36 volts for? A boat? Is it worth converting the loads/system to 24 or 48 volt given the small amount of 36 volt devices are are out there now-a-days?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 36 Volt Battery Charger

    chris,
    adding to bb's further explanation you will find that 12v batteries really aren't exactly 12v all of the time either. to charge some batteries the battery voltage may have to reach as high as 14.8v for some batteries. others may need around 14.4v as type and manufacturer do play a part in that. they may rest fully charged around 12.7v or so again depending on type and manufacturer. as you use the batteries, the voltage will drop and most batteries are considered dead when their at rest voltage is 10.5v. nobody wants to get that low as it will kill the battery lifespan.
    also note that battery temperatures also play a role in how much voltage they will need to attain. in addition there will always need to be a higher charge voltage for if there isn't a voltage differential then no current will pass. you can think of batteries in those 12v multiples, but be careful as a 12v pv is not really 12v as that more or less refers to it's application voltage.
    i hope our further explanations help you to understand better.