Pulling 12 volts from a 24 volt bank

littleharbor2

I was wondering what your thoughts are on tapping 12 volts from a 24 volt bank of batteries? The scenario is a simple 2-12 volt series wired bank of batteries. You can pull 12 volts from one of the two batteries, and I know this isn't the best thing to do due to the imbalancing of the batteries voltage. What I am proposing is tapping BOTH batteries and paralleling that. Seems the load on the batteries would be even at that point. I must be missing something here or this scenario would be a common and simple way of getting 12 volts out of a 24 volt module/battery bank setup.
Your thoughts are much appreciated. Little Harbor 2

vtmaps

Re: Pulling 12 volts from a 24 volt bank

littleharbor2 wrote: »

I was wondering what your thoughts are on tapping 12 volts from a 24 volt bank of batteries? The scenario is a simple 2-12 volt series wired bank of batteries. You can pull 12 volts from one of the two batteries, and I know this isn't the best thing to do due to the imbalancing of the batteries voltage. What I am proposing is tapping BOTH batteries and paralleling that. Seems the load on the batteries would be even at that point. I must be missing something here or this scenario would be a common and simple way of getting 12 volts out of a 24 volt module/battery bank setup.
Your thoughts are much appreciated. Little Harbor 2

What you are missing is the explosion that will occur. You can't hook up two 12 volt batteries in series (24 volts) and parallel (12 volts) at the same time.

You need a Dc to DC converter. A company called 'Solar Converters' makes high quality ones.

--vtMaps

BB.

Re: Pulling 12 volts from a 24 volt bank

The "practical" ways of taking 12 volts from a 24 volt battery bank:

1. Wiring 12 volts from one battery: If loads are very tiny--It can be done. Larger loads will "unbalance" the batteries. The tapped battery will always be under charged or the untapped battery will always be over charged (you have to over charge the untapped battery to get the tapped battery fully charged--very bad for either battery).
2. DC to DC converter: 24 volt battery to 12 VDC output... Common solution. Really not much different that using a 24 volt AC inverter and powering loads at 120 VAC and skipping the whole 12 VDC output--Many times, 120 VAC just ends up being easier to use everywhere vs 12 VDC appliances (issues of 10.5 to 15.5 volts from a flooded cell battery bank and many "12 volt" loads not really living well in those ranges. Higher end DC converters are around the same price as 120 VAC inverters (in many cases).
3. DC to DC converter: For some vehicles that need both 24 volts for engine starting and 12 volts for other loads--There are DC to DC converters that are designed to connect from low bank to high bank. They are "bi-directional" isolated output converters. They keep the high/low bank battery voltages "identical"---You can discharge/charge either high 12v or low 12v bank or 24 VDC and the converter will keep the charge on both banks identical. Again, not a cheap solution.
4. 24v to 12v charge controller: Basically get a 24 volt charge controller (you can use some MPPT charge controllers for this function) to charge a 12 volt VDC battery bank. Advantage is you have a full off grid 12 volt battery+charge controller--Handles surge current well and is a backup to 24 volt power. Downside is you have an "extra" 12 volt battery to maintain.

Almost any of the above solutions are similar in cost and complexity to just getting the "right sized" 24 VDC to 120 VAC inverter. And running all of your loads from 120 VAC. Most energy star appliances can be just about as energy efficient as their DC counterparts--And many times are much cheaper and easier to have repaired than the DC versions. And all you need to do is make the solar array+battery bank ~20% larger (to account for the 85% efficient AC inverter).

-Bill

littleharbor2

Re: Pulling 12 volts from a 24 volt bank

So if you tap into one battery you get 12 volts. Now you tap into the other battery and pull an equal amount of power out of it would you not be discharging the two batteries at the same rate and not be causing the problem of discharging the bank unevenly? I'm missing where the "Explosion" happens.

BB.

Re: Pulling 12 volts from a 24 volt bank

Yes--If the loads are naturally balanced--Yes, you can do this pretty well.

The dead short was read from this:

What I am proposing is tapping BOTH batteries and paralleling that. Seems the load on the batteries would be even at that point.

You cannot "parallel" the wiring. Each load would require its own +/- leads. You cannot "Parallel" high+low bank batteries by just wiring them together.

You can use isolated output DC to DC converters to "parallel" the high+low bank of batteries...

To connect (for example) two 12 volt light bulbs. Connect the low bulb to ground and +12 volts. And the second bulb to +12 and +24 volt leads. If the bulbs are "balanced" current--Then there would be close to zero current flowing through the +12 line back to the battery bank.

Some loads (like filament lamps) can be connected in series (like a pair of 12 volt lamps)--Because they naturally balance their current flow.

Other loads (motors, LED/CFL lamps, other non-linear loads)--Their I*V curves do not generally allow them to connect in pure series (without the middle voltage connection). Generally they will fail because one unit will see close to 24 volts when the other is near zero volts.

-Bill

Blackcherry04

Re: Pulling 12 volts from a 24 volt bank

littleharbor2 wrote: »

So if you tap into one battery you get 12 volts. Now you tap into the other battery and pull an equal amount of power out of it would you not be discharging the two batteries at the same rate and not be causing the problem of discharging the bank unevenly? I'm missing where the "Explosion" happens.

The magic word you said was " Parallel ". If you try to parallel the output you'll get big sparks. You can lift the interconnect between both batteries and be ok. You can have two separate loads on each battery that do not share any common wiring. Bottom line, do it right and use a converter.

AuricTech

Re: Pulling 12 volts from a 24 volt bank

Looking at a diagram of a pair of batteries wired in serial, then thinking about drawing current from them in parallel (while they're still wired in serial), leads me to conclude that, in effect, the positive terminal of one battery would simultaneously be connected to both terminals of the other battery. I'm pretty sure the technical term for that is either "explosion" or (at very least) "what could possibly go wrong?"....

littleharbor2

Re: Pulling 12 volts from a 24 volt bank

I am understanding now where the problems lie. Thanks to all for your timely input. This forum is the best source of knowledge around.