Charging 24 volt batteries from a 48 volt array

[Rabrsniver]

Is there any harm in charging 24 volt batteries directly from a 48 volt solar array? My charge controller has failed, yet I want to try and charge my batteries while it is being repaired. The charge controller is an Outback MX-60 and it automatically steps down the voltage.

If I "babysit" it and make sure the battery voltage doesn't go above about 25 volts, can I use the 48 volt input directly from the panels, or will this high input voltage be too much for the batteries to handle? I'd rather not re-wire the panels to 24 volts if I can help it.

Thanks,

John

[whatsnext]

A little more info would help. What is the AH rating of your battery pack and how large is your panel in watts. As long as you babysit though you should be fine. I would probably look for something more like 27 volts though. Your batteries won't see the 48 volts because they are likely 'more powerful' then your panels unless you have a small bat and a big panel. Is there any way to split your panel so that you could make better use of it?

John......

[Rabrsniver]

The amp hour rating of my 12 Trojan L-16HC's is 1185.

I have 660 watts of solar. (4 Sharp 165 watt 24 volt panels wired in series and parallel).

I COULD re-wire them as straight 24 volt, but it is kind of a pain for what will probably be about a week or so until the charge controller is fixed.

The problem is that my inverter also went bad. The generator will take care of the 120 volt loads, but my water system is run directly from the 24 volt batteries.

It has not been a good summer:

First the generator died on me. (bad voltage regulator)

Then the charge controller freaked out and quit working.(Outback MX-60, which was a nice unit while it was working)

Then I found a dead cell in one of my batteries.

Now the Trace 4024 inverter had a Heatsink Overtemp error and fried some of the MOSFETs.

And of course its been one problem after another, if not more than one at a time.

The only positive is that I am basically out of things that can fail. (I hope)!

John

[ghurd]

Babysitting should be fine.

I'd let them go to 29V... if I was babysitting.

G-

[whatsnext]

Your cells will only produce 330 watts into a 24VDC bank so that's about 14 amps. That's about a C85 rate. Very slow, which is good in this case. Babysit them and you'll be just fine and sorry to hear about your stuff failing. The good news is that you bank will start charging about one minute after dawn.

John........

[whatsnext]

BTW. You might find that you like unregulated charging. I'd give it a go for a bit before you buy another controler. Many here would disagree with this little tidbit but you're being forced to try it.

John...

[Experimental]

      Sorry to hear of your problems John -- sounds like you have (had??) a very nice system !!

     If you can aquire a "load dump controller" -- you won,t have to baby sit that charging action..

    A lot of us use them on our wind gins, and you can get free plans off the Scoraig wind site -- but you might try  this sites founder , for that item !!

However, I realize, you need it NOW..  ( they are on ebay too)

     Hope you get it all sorted out, and this week is better ... Bill H..

[geoffd]

I would say it would be ok.  The MX-60 is an MPPT charge controller, so it is designed to get the best IV curve characteristics from a panel, so it will adjust the voltage to the best point anyway.  If the System voltage is set for 24 volt then a higher input voltage wont matter.

I am looking at doing this too as my 24 volt PV array is 100 meters from my battery room and to reduce to voltage drop on the cable I want to wire the array as 48 v into a 24v battery.  An MX-60 is on my shopping list.

Cheers

Geoff

[wpowokal]

Ok let me make a fool of myself, cause me thinks something is rotten in Rome.

1. since solar regulators aparently short the output when regulating there is obviously no harm to be done to the panels. And as you are already babysitting you batteries no harm to them.
   
2. I fail to understand why with a 24v system the solar panels are wired for 48v. My understanding of MPPT is your best return on panels would be wired for 24v, letting the MPPT optimise the charging.
   
3. my Shell SP140 panels (24V) list the max open circuit volts as 42.8. Voltage at load vmpp as 33v.
   
   

So this tells me that if I had a MPPT, which I don't, neither a controller, I would still only need them connected as 24v which they are.

Perhaps your panels are vastly different to mine or maybe you have been misinformed on the MPPT.

I googled Sharp 165w panels, open volts 44.4, MPPV 35.4V, MPPI 4.95A.

Me thinks you may be missing out on charge, like 50% of available, there again a work mate once said "it's best to have people think you are an idiot than to open your mouth and confirm it".

allan down under

[ghurd]

Some controllers short the PVs (shunt).

Some controllers open the circuit (series).

G-

[boB]

The MX60 regulates the output to the battery by changing the PV input voltage to somewhere between max power point and PV open circuit voltage, or completely off (disconnected)

There are three typical advantages to wiring the PV for a higher nominal voltage than the battery.

One is that at dawn and dusk, you can harvest a bit more energy from the array.

Two, is that if you have partial shading and the max power point voltage goes down, it is still above the battery voltage.  This also helps the energy harvest.

The big advantage, most of the time, though, is that there is less wire loss in the PV to controller lines since the current is lower. This is an I^R relation, so if you double the input PV input voltage, you reduce your wire loss by 4.

  Or, ppl can save money for very long wire runs by being able to use smaller wire.  Sometimes this is all it takes to more than pay for the MPPT charge controller.

Have a day and 1/2 !

boB