battery voltage drop/inverter alarm

System

I have 2 - 1.5kwh batteries feeding 1350 watt inverter. I plug in 700W heater. Voltage drops from 12.2 to 11.6 at terminal and inverter alarm goes off. Turn off heater and batter measures again 12.2 I'm new to this. It seems inverter alarm signal is set too low? Heater is running fine here, its just the sound of the alarm bothering me. This seems like it would be normal to see this voltage drop? Any insights here would be helpful.
Oscar

icarus

Re: battery voltage drop/inverter alarm

A 700 watt load on a (comparatively) small battery is huge. Please give some more details on the battery, but assuming it is a 12 volt configuration 700 watts is 58 amps. A battery can only sustain that big a draw for a short time.

How have you calculated your "1.5" kw battery? Most batteries are rated in Amp/hours at a prescribed rate. A battery might be able to deliver 1.5 kw before being drained dead, over say a 24 hour period. Do that same draw in 2 hours, and it won't deliver the same performance.

Tony

PS Welcome to the forum, there are some very smart folks here who are very willing to help.

T

PPS One might ask the question, why are you using a battery to power a heater? Electric heaters are huge draws for PV and battery systems and best avoided.

westbranch

Re: battery voltage drop/inverter alarm

Oscar, that starting voltage is very low .... if the battery is fully charged it should be 13.2 V not 12.2. Please clarify.

HTH

icarus

Re: battery voltage drop/inverter alarm

Good catch Eric, I missed the voltage.

I might suggest that the OP read the following:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Tony

BB.

Re: battery voltage drop/inverter alarm

What is the "resting voltage" of the battery bank (should be around 12.7 volts after 2-3+ hours of no charging/no discharging around 77F).

Another question is what is the length of the cabling and what gauge is it...

At low voltage cutoff (typically around 11.5 to 10.5 volts for most inverters) there is not a lot of head room for wiring/switches and low battery voltage...

If the battery is 11.6 and the inverter is 10.6 cutoff--The voltage drop for 5' of cable would be:

• 700 watts * 1/0.85 inverter eff * 1/10.6 volts = 77 amps
• 77 amps @ 5' of 10 awg cable would give you ~0.9 volt drop

So, it is very conceivable that you have around a 1 volt drop and your inverter is simply at cutoff voltage (depending on cable length and awg).

That is a pretty heavy load for a 12 volt battery bank of that size...

If these are 2x1.5kW rated flooded cell batteries (at 20 Hour rate), Normally a good discharge rate is around C/8 (~12.5%) rate of discharge. And surge rating would be around C/2.5 (~40%)

• 3,000 WH * 0.125 rate = 375 Watts recommended continuous discharge
• 3,000 WH * 0.40 rate = 1,200 watts maximum recommended surge current

If these batteries are AGM--Some batteries are rated as high as C*4 current rating (UPS type batteries which are designed for 15 minutes of backup power).

But, I am not sure how long the batteries would last if this was a typical daily cycle (UPS usage seems to last only a few years of battery life).

Using batteries is pretty expensive for heat--is there any alternatives available (propane, etc.)?

-Bill

RCinFLA

Re: battery voltage drop/inverter alarm

12.2 vdc open circuit is a nearly dead battery to start with.

If you have done the math correct, I assume your comment about a 1.5 kWH battery means a 125 AH, 12v battery. Maybe two Walmart Everstart MAXX-29 series.

Alarms on inverters are usually set for 10.5-11 vdc.

For that sized inverter, depending on total wire length, should be using at least #4 wire, preferred #2 wire.