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Old 02-07-2014, 08:49 AM   #1
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Question about building a battery bank and using 24v system

Hello,
had an earlier thread asking about battery bank questions.
I have another question now, I was going to build a 24v system and use a 24v inverter because it's more efficient and the wire will be less expensive.

I bought a used Onan 5500 LP generator and it doesn't have a starting battery.
I am also gong to get en electric tongue jack.
With the 24v system I will need to buy another battery and put it on a charger every night.

Would it be simpler to just buy the 12v inverter (24v is the same price) and spend the extra money for the larger wire and not have to buy another 12v battery to start the generator and power the jack?

I just bought the batteries and I think I got a good deal and bought four 12v 200ah AGM for $1,260 shipped. I cannot used lead acid since the only room for batteries is inside the concession trailer that's being built.
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Old 02-07-2014, 08:53 AM   #2
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If you do things in series, you can run the wire from the battery that is 12V to the genset.
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Old 02-07-2014, 08:57 AM   #3
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I believe you could also connect the generator starter and power jack to the 24 volt battery bank with a 24VDC to 12VDC step down transformer supplying sufficient current (150 amps?).
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Old 02-07-2014, 08:59 AM   #4
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A very few folks have converted to 24Vdc house battery banks. As you mention there are a few pluses including the fact that at twice the voltage the current is cut in half so smaller wires can be used, however the downside IMO far out weights the upside. Most all your DC equipment is designed for 12Vdc. You could use DC to DC Boost/Buck conversion which adds complexity and more chances for failure.

For me I stuck with 12Vdc and large wires as needed - welding cables are great.

PS - Those AGM's are indeed Lead Acid batteries just sealed up tight as compared the vented GC Batteries I use
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Old 02-07-2014, 09:04 AM   #5
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As related in the OP's other thread, it is a concession trailer with a 24 volt system supplying 110V AC through a 24 volt inverter or the generator. No 12 volt draws, except the generator starter and power jack which I would suspect will be used intermittently.
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Old 02-07-2014, 09:32 AM   #6
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I believe you could also connect the generator starter and power jack to the 24 volt battery bank with a 24VDC to 12VDC step down transformer supplying sufficient current (150 amps?).
Thanks, that may be a way. Would save me a $75 battery but buying a $100 converter. But at least I wouldn't have to charge a separate battery. Just the shore power plug at night to charge the battery bank.

If I did something like that, I would connect it between the battery bank and inverter right?
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Old 02-07-2014, 09:47 AM   #7
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Thanks, that may be a way. Would save me a $75 battery but buying a $100 converter. But at least I wouldn't have to charge a separate battery. Just the shore power plug at night to charge the battery bank.

If I did something like that, I would connect it between the battery bank and inverter right?
NO. If you did that, you would step the whole system back down to 12V and lose all the benefit of going to 24V in the first place. You would attach it in parallel with the inverter (just to make a step down to 12V to start your generator)

I'm still of the opinion that a simple connection at the 12V level in your battery bank to obtain 12V to occasionally start your generator is the best way to go. The effect on the battery bank (or any individual battery) will be negligible, in spite of what others claim.
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Old 02-07-2014, 12:29 PM   #8
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I talked to the inverter store and they said the 24v is more efficient and their 12v inverter is very power hungry they suggested the 24v.

They also said that tapping into one of the batteries to start the generator and operate the tongue jack would have extremely minimal negative results on the battery bank and it wouldn't cause any harm at all to the battery.
They said if I was running anything for a length of time then use a separate battery but for a few seconds use a few times a day would be perfectly fine.

If I went this route I could connect the wires to a different battery every few weeks or so and hopefully keep each of them exactly the same as the others.
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Old 02-07-2014, 12:52 PM   #9
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I talked to the inverter store and they said the 24v is more efficient and their 12v inverter is very power hungry they suggested the 24v.

They also said that tapping into one of the batteries to start the generator and operate the tongue jack would have extremely minimal negative results on the battery bank and it wouldn't cause any harm at all to the battery.
They said if I was running anything for a length of time then use a separate battery but for a few seconds use a few times a day would be perfectly fine.

If I went this route I could connect the wires to a different battery every few weeks or so and hopefully keep each of them exactly the same as the others.
It would probably involve repositioning the batteries within the bank, rather than simply attaching across a different battery, as you will need to maintain the same reference to ground between the generator, batteries and inverter.
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Old 02-07-2014, 12:55 PM   #10
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It would probably involve repositioning the batteries within the bank, rather than simply attaching across a different battery, as you will need to maintain the same reference to ground between the generator, batteries and inverter.
Thanks, I was thinking of just having a few feet of wire extra in the battery compartment and just unscrew the connectors and put the leads onto a different battery. But if the system 24v gets disconnected somehow by doing that then I wouldn't want to do it.
When you say maintain the same reference to ground, I don't know what that means.
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Old 02-07-2014, 12:58 PM   #11
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So why not go with a 48Vdc inverter and reduce the current even further?
You have 4 batteries put them all in series and you are good to go
48V is far more common than 24V
The vast majority of residential installations use 48V plus 100% of the telecom industry uses 48V

Me I would NOT ever tap off a battery that is in a series string
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Old 02-07-2014, 02:08 PM   #12
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Thanks, I was thinking of just having a few feet of wire extra in the battery compartment and just unscrew the connectors and put the leads onto a different battery. But if the system 24v gets disconnected somehow by doing that then I wouldn't want to do it.
When you say maintain the same reference to ground, I don't know what that means.
I was just thinking that if the case of the inverter is grounded to a "common ground" and the generator starter and frame are attached to that "common ground", you need to attach to a battery, within your array, that has 12V on the positive anode with reference to that common ground. Attaching across a battery that is wired into the array with 12V on the negative and 24v on the positive, would cause a spark shower to say the least.

Folks will continue to come out of the woodwork advising against tapping off the necessary 12V from your 24V bank to occasionally start your generator. When they offer to use their money to purchase and install the additional battery, and necessary 12V converter/charger, take them up on the deal.
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Old 02-07-2014, 03:01 PM   #13
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Me I would NOT ever tap off a battery that is in a series string
Why is that? Just curious, we're all trying to learn.
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Old 02-07-2014, 04:52 PM   #14
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So why not go with a 48Vdc inverter and reduce the current even further?
You have 4 batteries put them all in series and you are good to go
48V is far more common than 24V
The vast majority of residential installations use 48V plus 100% of the telecom industry uses 48V

Me I would NOT ever tap off a battery that is in a series string
The place that I'm buying the inverter doesn't have a 48v inverter. The best price I found for a 24v pure sine 4,000 watt is $1,300.
If I can find a 48v pure sine for that price I would like to look into it.
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