What size battery to power a battery charger

[MarkSolar]

I have a storage building for tractors and equipment, no power in the building. I need a solar system to power the 120v battery charger that's integrated into a battery operated boom lift. The lift has 4 of these Trojan batteries. I never let them go below about 1/2 charge. I'm trying to figure out what size battery I need on a solar system to run the lift's battery charger long enough to recharge the batteries.

If I work from each of the 4 lift batteries being 1.5kWh, that's a total of 6kWh, and if they're half charged that means I have to put 3kWh back into them. Is that the right way to think about this? I've put a meter on the system when it's charging, it briefly draws 9A@120v, then quickly drops to about 7, then 6 and finally 4A after an hour or so. It takes 6-8 hours to fully charge from ~1/2 discharged state.

I can always choose when I want to do the recharging, so I can wait until the morning of a sunny day and plug it in. So the panels can help the battery with the charging. So if I get a battery that can deliver 3kWh in 8 hours, that's 375W which is 30A. So what kind of battery do I need to deliver 30A for 8 hours and only be about 1/2 discharged so I don't damage the battery? I guess in theory if I have a 375W panel I don't need any battery, so I'm not sure how to think about this.

 

[Bobert]

I have a storage building for tractors and equipment, no power in the building. I need a solar system to power the 120v battery charger that's integrated into a battery operated boom lift. The lift has 4 of these Trojan batteries. I never let them go below about 1/2 charge. I'm trying to figure out what size battery I need on a solar system to run the lift's battery charger long enough to recharge the batteries.

If I work from each of the 4 lift batteries being 1.5kWh, that's a total of 6kWh, and if they're half charged that means I have to put 3kWh back into them. Is that the right way to think about this? I've put a meter on the system when it's charging, it briefly draws 9A@120v, then quickly drops to about 7, then 6 and finally 4A after an hour or so. It takes 6-8 hours to fully charge from ~1/2 discharged state.

I can always choose when I want to do the recharging, so I can wait until the morning of a sunny day and plug it in. So the panels can help the battery with the charging. So if I get a battery that can deliver 3kWh in 8 hours, that's 375W which is 30A. So what kind of battery do I need to deliver 30A for 8 hours and only be about 1/2 discharged so I don't damage the battery? I guess in theory if I have a 375W panel I don't need any battery, so I'm not sure how to think about this.

If you can get a hold of a wattmeter and plug your charger into that you should be able to get the exact amount of watts that you need

 

[time2roll]

Close to 300 amp-hrs would be my suggestion. Also skip the 12vdc--> 120vac --> 12vdc conversion and get a DC-DC charger.

https://renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
Also available in 40 amp and 60 amp. Shop around.

 

[MarkSolar]

If you can get a hold of a wattmeter and plug your charger into that you should be able to get the exact amount of watts that you need

I have a watt meter but can never remember to use it when the batteries are discharged. I'm paying more attention now that I'm getting ready to spend money.

 

[MarkSolar]

Close to 300 amp-hrs would be my suggestion. Also skip the 12vdc--> 120vac --> 12vdc conversion and get a DC-DC charger.

https://renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
Also available in 40 amp and 60 amp. Shop around.

I've thought about using a 12v charger but that would require I disconnect the batteries from the lift system to charge them because I'm not going to connect an external battery charger to the output side of a circuit board that probably costs $1000 if I fry it.