Can a 24v inverter run from half of a 48 volt bank?

esniper · March 2017

I have 24 x 12 volt Unigy AGM batteries. I have two banks of 12 batteries each setup as 24 volts. I want to connect the two banks in series to create 1 big 48 volt bank. I have a 24 volt Samlex 1000 I wanted to connect to one of the banks. Would that work or would the samlex see the input as 48v instead of 24v? I don't have it setup yet otherwise I would use a meter to see if I hooked up to the pos and the neg of one 24v bank to see what it sees after it is connected in series to the other 24v bank.
Thanks

esniper · March 2017

As a mini test I hooked a meter on 1 12 volt battery in a 24v config and as expected I got 12 volts. So in theory if I had 2 x 24 volt batteries hooked in series for 48 volts I could hook my 24 volt inverter to the pos and neg leads of the 24 volt bank and draw off half the bank. The other concern is my charge controller is going to be charging everything @ 48 volts. I wonder if that would cause a problem to the 24 v inverter?

mcgivor · March 2017

esniper said:

I have 24 x 12 volt Unigy AGM batteries. I have two banks of 12 batteries each setup as 24 volts. I want to connect the two banks in series to create 1 big 48 volt bank. I have a 24 volt Samlex 1000 I wanted to connect to one of the banks. Would that work or would the samlex see the input as 48v instead of 24v? I don't have it setup yet otherwise I would use a meter to see if I hooked up to the pos and the neg of one 24v bank to see what it sees after it is connected in series to the other 24v bank.
Thanks

Don't even consider center tapping a 48v bank to power a 24V inverter, or any other voltage, you WILL destroy the batteries, it will work, just not for long. What will happen is the tapped batteries will be depleted, the charger will not fully charge them, they will devlop sulfation and suffer a premature death.

You say you have 24 ×12V, are these small batteries you're attempting to string parallel series?

esniper · March 2017

I will probably run everything @ 48 volt. One reason I am asking to see if it is possible. I can wire two banks @ 24 and connect them in series to make 48v. I can revert to 24 if the UPS I plan on running as an inverter blows up. I have 4 x 3000W APC UPS I am going to try and run @ half power and cool them with a fan or two. I will see how one or 2 lasts. If it nukes one I can then easily change back to 24v. My charge controller is 12, 24 or 48 so that too is easily altered.

mcgivor · March 2017

To be honest, I've no idea what it is you're trying to do, for example, and I quote you " As a mini test I hooked a meter on 1 12 volt battery in a 24v config and as expected I got 12 volts", . Basically you choose a nominal voltage, everything must use the same nominal voltage, ie. 24V charge controller, 24v battery, 24v inverter, trying to configure any other way, 48v battery with 24V inverter?? is a recipe for disaster, in my opinion.

jonr · March 2017

The only case where I would consider tapping a string is if a) you have a balancer installed and b) loads (from the tap) will be low or intermittent.

http://www.ebay.com/itm/Battery-equalizer-HA02

Estragon · March 2017

In a pinch it will work.

Presumably you have 24v charge source as well as the inverter. If you do end up tapping 24v off the 48v bank, it might help to recharge the 24v segment on its own too. The main problem with discharging only one part of the string is that in recharging the whole bank, the unused batteries are getting overcharged while the segment used may still be undercharged.

Keeping a bunch of parallel batteries balanced is a challenge, esoecially with AGMs. Deliberately putting it way out of balance isn't a great idea, but if you know you'll need to deal with the resulting imbalance promptly maybe you could get away with it.

I have had to partly charge a 12v segment of a 48v bank so I know it works. In that case I just needed to get voltage up enough to wake inverters so I could charge at 48v from a generator. It's not something I would want to do regularly.

mcgivor · March 2017

jonr said:

The only case where I would consider tapping a string is if a) you have a balancer installed and b) loads (from the tap) will be low or intermittent.

http://www.ebay.com/itm/Battery-equalizer-HA02

The key words are low and intermittent, 1000W is not low, by any stretch of the imagination, the balancer can compensate for small loads in the milli amp regon, but not in much more, futhermore having a excellent fundemental understanding of electrical behavior is paramount in experimental endeavors, which is the basis for not recommending such an endeavour in the first place , most will result in disappointment, but in the end it's all educational, we/most of us have paid the price of education.

jonr · March 2017

Low OR low duty cycle intermittent and there is no doubt that it's best to not do it at all (especially if you don't know what you are doing). The balancer listed claims 10A of balancing. Not sure I quite believe that, but it's not milli-amps.

Dave Angelini · March 2017

Pretty much a bad idea but offgrid there are times where something weird like this has to happen for a short period.
If you do it, you will need to alternate between taps so the bank stays balanced.

mike95490 · March 2017

Absolutely it will run your inverter just fine. And it will Absolutely destroy your battery bank in a couple weeks

bill von novak · March 2017

jonr said:

The only case where I would consider tapping a string is if a) you have a balancer installed and b) loads (from the tap) will be low or intermittent.

I'll add a third condition: only if you also have long periods of zero load so the battery can rebalance itself. If there is always an unbalanced draw, that section of the battery will always be at a lower state of charge - so half the battery is either undercharged or overcharged.

jonr · March 2017

If you have a balancer in place and the load you are drawing from the substring is less than what the balancer can move, then the substring will never be at a lower state of charge than the other batteries in the string. Even if the load is continuous.

Here is another, name brand balancer. Only .7A but it has alarms (perhaps an irritation for brief large loads).

https://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf

mcgivor · March 2017

The intent of these battery balancers is to maintain minor differences in battery chemistry, using one to intentionally discharge by connecting a 1000 watt inverter over 24V of a 48V string, would be like using a band aid to treat a compound fracture.

BB. · March 2017

Just to give you an idea what could be done (for 12:24 VDC system, such as a Bus:RV conversion):

https://www.waytekwire.com/datasheet/extend battery life.pdf

The "higher function" equalizer basically tries to hold "equal voltages" across both the "high 12 volt" and "low 12 volt" batteries. And the equalizer/balancer is actually bi-directional. Most applications charge the battery bank with a 24 volt source (alternator/wind/etc.) and the low battery is tapped for 12 volt loads. However, the bi-directional type is capable of having a 12 volt charging source connected to the "low battery", and it will take power from the low battery and use it to charge the high battery too (i.e., the equalizer attempts to hold the dame voltage across both batteries, whether charging or discharging).

So far, I have not seen any off the shelf 12:48 volt systems. You can get 12:12(24) volt balancers (Solar Converters Inc. from our host). These are isolated 12 volt to 12 volt bi-directional switching power supplies. You can put them across multiple batteries and (I think) get a 12 volt system out of it--However, the expense and complexity--I would highly suggest that you just choose a voltage (12/24/48 volt) and stick with it. I don't that you could justify the costs/complexity of trying to make your own 12:48 volt balancer.

-Bill

bill von novak · March 2017

jonr said:

If you have a balancer in place and the load you are drawing from the substring is less than what the balancer can move, then the substring will never be at a lower state of charge than the other batteries in the string. Even if the load is continuous.

Nope. As always, Ohm rears his ugly head. You will see a lower voltage across the battery being drawn down due to the finite resistance in the balancer leads. Balancers are not intended to be used this way - which is why you need to allow time at no load for the battery to rebalance.

You can always try it, of course.

Here is a name brand balancer.

Yep. Now read the data sheet:

"When the charge voltage of a 24V battery system increases to more than 27,3V, the Battery Balancer will turn on . . ."

Which means it will be off during most discharges. So for this balancer you have to allow even MORE time - and can never discharge the lower bank more than about 50%, or 25% of your total bank capacity.

jonr · March 2017

> You will see a lower voltage across the battery being drawn down due to the finite resistance in the balancer leads

All balancer designs I've seen worked on a pulsed basis. They measure voltage difference while moving no current, then, if needed, move a pulse(s) of current. There is no voltage drop across the balancer leads while the measurement is being done. Resistance in the balancer leads doesn't matter.

I agree that the Victron design is odd. The HA02 sounds better and gets good reviews. On the other hand, it's low cost, non name brand and I haven't seen rigorous tests...

Interesting, the models that Bill mentions go up to 50A. With good efficiency and low ripple. And Waytekwire discusses continuous loads less than what the balancer/equalizer can move (ie, it's a listed use case).

http://www.solarconverters.com/products/desulphators/battery-equalizer/eq-1224-50/

bill von novak · March 2017

jonr said:

All balancer designs I've seen worked on a pulsed basis. They measure voltage difference while moving no current, then, if needed, move a pulse(s) of current.

Exactly. They don't work until there's an imbalance. If the imbalance is caused by a transient change in battery state (i.e. you just filled the battery) then it runs until the voltages are equal again. If the imbalance is caused by a steady draw, then it runs constantly, since one segment of the battery is always at a lower voltage.

jonr · March 2017

And fortunately, the voltage difference detection limit on the balancer is well below the level that is significant (ie, effectively the same state of charge). With or without a small continuous load.

bill von novak · March 2017

jonr said:

And fortunately, the voltage difference detection limit on the balancer is well below the level that is significant (ie, effectively the same state of charge). With or without a small continuous load.

You are completely missing the point here.

But it's your money. Do what you like.

Can a 24v inverter run from half of a 48 volt bank?

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