# Help sorting this out!

moonbow
Posts:

**9**Registered Users ✭✭
Good Morning, I have a simple ( I think) question that I cannot
quite get straight in my head. Here goes...
In my RV, I have a nice solar system comprised of a roof panel, charge
controller and:
(2) - 6 volt 220 AH AGM Batteries wired in series to make a 12V 220AH
bank.
(1) - 3000W Pure sine Wave Inverter.
I also have a generator to use when needed but my question is this.
I have a 6 gallon water heater that draws 1550 Watts and 14 Amps. When I
want to heat water, I bypass the solar system and turn on the generator
for 30 to 40 min . For curiosity's sake, I decided to run the Water
heater off the batteries / inverter. When I did this ( with a fully
charged battery bank), my charge controller read that the battery bank
voltage was dropping to 11.8v while the inverter was reading a 1560Watts
of output.
I wasn't sure what to make of this so, after about 30 sec, I stopped
using the batteries for the WH . The voltage reading on my bank
rebounded to 12.7 as soon as the inverter was turned off and the load
removed.
While not totally necessary (because I have a generator) I would think
my fully charged battery bank would be able to handle a 14 Amp load for
30 to 40 min. Is it normal and OK to see the battery bank voltage drop
like this during a heavy load draw?
Thanks in advance

0

## Comments

4,709Solar Expert ✭✭✭✭9Registered Users ✭✭2,852Registered Users ✭✭✭✭✭The 220amp-hour capacity is likely a 20 or 24 hour rating. At a discharge rate approaching the 1 hour rate, the effective capacity is greatly reduced, so the bank may behave more like a 120ah capacity. My guess is by the .5hr point, the voltage would have sagged to the point the inverter would drop the load to protect itself.

You can get away with big loads for short times (eg microwave), but for longer periods you get into a sort of self-reinforcing death spiral.

Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer

Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

4,709Solar Expert ✭✭✭✭27,931Super Moderators, Administrators adminAlso, remember that Power=Voltage*Current... You are pulling 14 amps @ 120 Volts for your electric water heater. At 12 volts, you are drawing from the battery bank:

And for worst case electrical design:

That is a lot of current to pull from a 220 AH capacity (20 hour rate) lead acid storage battery.

Realistically, an 8 hour discharge rate is about what I would suggest drawing from a standard deep cycle lead acid battery... In terms of AC Watts, that would be:

Now--You do have an AGM battery bank. AGM batteries are known for supporting very high current draw... 1 hour discharge or one brand even a 15 minute discharge (4x rated battery capacity).

HOWEVER--Drawing high current from AGM batteries can damage the batteries (UPS batteries are typical AGM or GEL batteries--And are replaced every 1-2 years or after a "significant" discharge/power outage or two).

Also, if you are going to draw such high currents from your lead acid (or AGM) batteries at such low voltages (12 volt battery bus), you need very heavy and short cables from the AC inverter to the battery bank. For example, 0.5 volt drop 3,000 Watt 12 volt AC inverter:

- 3,000 Watts * 1/0.85 AC inverter eff * 1/10.5 volt AC inverter shutdown voltage * 1.25 NEC wiring+breaker deratings = 420 amp rated wiring and branch circuit.

Using the NEC wiring table (relatively conservative):https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

You would need a pair of 4/0 cables in parallel for such high current draw (can do with smaller cables--This is a reasonably conservative recommendeation for DC bus cables in free air).

Then you want to have a maximum of 0.5 volt drop for your 12 volt wiring... Using a simple voltage drop accumulator and 2x 4/0 cable carrying ~4200 Amps, the maximum distance for the wiring would be:

5 feet (one way wire run--Some calculators would use 10 feet as round trip wire run)

Voltage drop:

0.52Voltage drop percentage:

4.35%Voltage at the end:

11.48

The short answer is I would not recommend a 1,680 Watt load on a 220 AH @ 12 volt battery bank.. It will support such a load for a relatively short time. However, in the long term it is very hard on the batteries and takes something like 1/2 of their capacity:I suggest that you don't take the battery below ~50% state of charge (for longer battery life). Also, when you discharge a battery with a very heavy discharge current (your discharge rate is getting close to 1 hour discharge rate)... Batteries are typically rated a 20 Hour Discharge Rate (for our needs). When you discharge at a 1 hour discharge rate, the apparent AH capacity of the battery will be less (say ~180-190 AH capacity or something...

-Bill

2,222Solar Expert ✭✭✭✭✭