12volt battery charging from a 48volt battery bank

[johngalt]

How would one charge a 12 volt battery from a 48volt battery bank?

Rather than discharge my good batteries at night just to run some LED lights I thought it might be better to keep a 12 volt battery charged on the side and let that one cycle instead. All my heavy work, when needed, would be done during the day when the sun is out.. Clothes washer, dish washer, etc. If my 48volt bank is topped off and I have no work to do, and there is still sun available, how can I use that power to charge a 12volt battery to run some LED lights at night?

I know I could down convert the 48v to 12v but I was trying to figure out a way to bypass the 48v all together for this purpose.

Is this possible, stupid concept, etc.?

[Naptown]

Look at the Morningstar tristar (not mppt)
You may be able to use this as a diversion controller to charge the 12V battery. Check with them on precisely how to configure this.

[johngalt]

Look at the Morningstar tristar (not mppt)
You may be able to use this as a diversion controller to charge the 12V battery. Check with them on precisely how to configure this.

Thanks - will do.

[dkpro1]

12 volt

Thanks - will do.

on my RV I run a 24 volt 1.2 kw system for main ac power
and a 120 watt to a 12 volt with a Sun saver duo keeps house battery and start battery
in good shape

[Mike90250]

I'll propose that ANY MPPT controller with 60VDC solar input, would be fine to charge a 12V battery with. You must match the charger size to the battery & load size. PWM controllers "short" the input to the output, and a 48V battery has no inherent current limit that a PV panel has, so I would only expect the PWM controller to work for a few minutes before it fries something.
Sorry Naptown, I disagree with you here.

[FloridaSun]

I'll propose that ANY MPPT controller with 60VDC solar input, would be fine to charge a 12V battery with. You must match the charger size to the battery & load size. PWM controllers "short" the input to the output, and a 48V battery has no inherent current limit that a PV panel has, so I would only expect the PWM controller to work for a few minutes before it fries something.
Sorry Naptown, I disagree with you here.

I was wondering about Naptown's response after reading this from the Tristar manual;

6.4.5 Minimum Diversion Load
The diversion load must be large enough to divert all the current produced by the
source (wind, hydro, etc.). This value is the maximum battery voltage times the
maximum source current.
For example, if a hydro source can generate up to 30 amps of current in a
nominal 48 volt system (60V maximum), the minimum diversion load size =
60V x 30A = 1,800 watts (for loads rated at 60 volts).

The diversion load must be large enough to divert ALL the current produced? Indeed that could tend to overload a 12V battery at times.

[Naptown]

I'll propose that ANY MPPT controller with 60VDC solar input, would be fine to charge a 12V battery with. You must match the charger size to the battery & load size. PWM controllers "short" the input to the output, and a 48V battery has no inherent current limit that a PV panel has, so I would only expect the PWM controller to work for a few minutes before it fries something.
Sorry Naptown, I disagree with you here.

My intended use was to use as a diversion load from the ARRAY not the batteries.

[Naptown]

I was wondering about Naptown's response after reading this from the Tristar manual;

6.4.5 Minimum Diversion Load
The diversion load must be large enough to divert all the current produced by the
source (wind, hydro, etc.). This value is the maximum battery voltage times the
maximum source current.
For example, if a hydro source can generate up to 30 amps of current in a
nominal 48 volt system (60V maximum), the minimum diversion load size =
60V x 30A = 1,800 watts (for loads rated at 60 volts).

The diversion load must be large enough to divert ALL the current produced? Indeed that could tend to overload a 12V battery at times.

Solar is different from wind and hydro as the use of a diversion load for wind and hydro is to keep the turbine from spinning to too high an RPM. With solar if there is excess energy it just sits there. The only reason to be able to divert 100% of the source with solar would be with an AC coupled system where the solar is not fed through a charge controller and batteries first (the internal battery charger is used)

[FloridaSun]

Solar is different from wind and hydro as the use of a diversion load for wind and hydro is to keep the turbine from spinning to too high an RPM. With solar if there is excess energy it just sits there. The only reason to be able to divert 100% of the source with solar would be with an AC coupled system where the solar is not fed through a charge controller and batteries first (the internal battery charger is used)

So on my 24V system would a tristar hooked up to a 12V batt produce 12v, with the 12V battery connected before diversion control input from solar panels/24V battery as per manual diagram. You say you would connect the diversion control to the array? not the battery? Do you mean before the primary CC? But doesn't diversion depend on a max voltage setting of the battery? LOL! I love this brain exercise. Please excuse my density at times. Takes me awhile to get this fully into my head.

[Naptown]

Suggestion
Contact Morningstar for their direction. I have read about this capability but have no direct experience with this and how it is configured.

[FloridaSun]

Suggestion
Contact Morningstar for their direction. I have read about this capability but have no direct experience with this and how it is configured.

No immediate plans to try any of this out but always interested in concepts.

My intended use was to use as a diversion load from the ARRAY not the batteries.

This sounds more like stacking a PWM with an MPPT, directing outputs to two dif batteries, not a diversion load.

[Sunking]

Well someone correct me if i am wrong here, but doesn't a diversion load just connect the input to output? If so. does not matter what voltage the battery is as long as the input voltage is greater than the battery voltage. Solar panels are current source and when operated below MPPT voltage is just a plain ole everyday current source all the way down to 0 volts.. If the IMP is 6 amps, that is what you get assuming 100% iradiance.

Of course doing so is real risky as you can easily over charge the battery with no control. Diversion loads are just places to dump heat, and to make a dummy load.

[FloridaSun]

getting back to johngalt's original question... seems to me a 48v to 12v reducer would be the best way to go unless you have room for a separate 12v system, panel and all. If dependent on excess amps from 48v panels to maintain a 12v battery, what would you do if there was no excess and that 12v was not topped off? Run the 48 to 12 reducer anyway.

[Naptown]

According to Morningstar as a diversion control it can be used to charge another battery.

http://www.morningstarcorp.com/en/support/library/TriStar%20Datasheet%20English.pdf

This may also require yet another controller to prevent overcharging the alternate battery.
Again Check with Morningstar for exact configuration.

[bcroe]

DC to DC

getting back to johngalt's original question... seems to me a 48v to 12v reducer would be the best way to go unless you have room for a separate 12v system, panel and all. If dependent on excess amps from 48v panels to maintain a 12v battery, what would you do if there was no excess and that 12v was not topped off? Run the 48 to 12 reducer anyway.

I would just hook a small DC-DC converter to 48V and get 12V out. Simple. Make it just big
enough for the (lights ?) load, could be turned off days by a photocell control. I'd use one with
an isolated output, at slight loss of efficiency. Bruce Roe