Battery Voltage Sag and Low Voltage Cutoff

[Pullin' Chocks]

We noticed when operating off grid from our Inverter that the battery voltage sags when under load. Remove the load and the battery voltage recovers to a higher voltage. We are operating from 4 each T-105 6 Volt batteries wired in series-parallel for a ~440AH 12V setup. Knowing that we only want to discharge ~50% our stop Voltage target is 12.1 Volts. (12.7 V being full charge). We set our Inverter low voltage cutoff at 12.1 V and noticed that the Inverter would auto shutoff when under load. For example our microwave oven draws ~17 Amps and pulls the full batteries down to ~11.9V when running.
My questions are is the battery voltage sag we are seeing about normal, and is it reasonable practice to lower the Inverter cutoff voltage to a lower value knowing the ambient unloaded battery voltage is higher than the loaded value?

[ARTWERKE]

You migh check wireing between batteries and inverter, if your microwave draws 17 amps(that sure sounds high) you're pulling 170+ amps off the batteries.it doesn't take much resistance in a connection to drop 1/2 volt with that kind of current.

[nothermark]

Use a voltmeter to check the voltage between battery post and battery terminal when under load. If you see more than 0.1 V check and clean the terminal to post connection. Also check post to ground and see if it sags as much as it does at the inverter input.

Another quick check on the DC side it to carefully run a finger over the connection points when under load or just done. 12 VDC will not shock somebody gut the heat generated by a bad connection will be noticeable.

FWIW when Infra Red sensors became commercially available power companies found that it was worth doing the same thing with cameras mounted in a survey vehicle. A lot easier to schedule a repair in HV lines rather than have them break in a storm. Power is the current squared x resistance so a little resistance goes a long way at 170 A. Normal connections probably get a bit warm. ;-)

[John Hilley]

Yes, you should set the inverter low voltage to a much lower point.

[wa8yxm]

I show 0.3 volt difference between the INVERTER terminals and the battery terminals under around 1200 watts of load.

You also have internal resistance in the battery to overcome.. This is one place where AGM batteries perform better. Lower internal resistance.

[wb7auk]

The low voltage cutoff should be 10 volts on the inverter.
The reason for this is the resting voltage of the battery and
the voltage under load allow the inverter to operate under load
without shutting off.

[pasdad1]

Originally Posted by Pullin' Chocks

We noticed when operating off grid from our Inverter that the battery voltage sags when under load. Remove the load and the battery voltage recovers to a higher voltage. We are operating from 4 each T-105 6 Volt batteries wired in series-parallel for a ~440AH 12V setup. Knowing that we only want to discharge ~50% our stop Voltage target is 12.1 Volts. (12.7 V being full charge). We set our Inverter low voltage cutoff at 12.1 V and noticed that the Inverter would auto shutoff when under load. For example our microwave oven draws ~17 Amps and pulls the full batteries down to ~11.9V when running.

My questions are is the battery voltage sag we are seeing about normal, and is it reasonable practice to lower the Inverter cutoff voltage to a lower value knowing the ambient unloaded battery voltage is higher than the loaded value?

To measure SOC (state of charge) you need a more sophisticated monitor like a Trimetric....simply measuring voltage is only partially accurate when the batteries are at rest and not under a charge or discharge.

[Pullin' Chocks]

Thank you for your thoughts & experience. I think we will start with lowering the cutoff voltage. When we have time we will try some of the voltage measurements recommended. The coach is fairly new and the wires between the battery and inverter are beefy, short and clean. We also torque the connections every few months to make sure nothing is vibrating loose. I don't expect too many issues with the wiring and connections. The performance has been stable as described.

[TQ60]

The setting is LVD or low voltage disconnect.

Industry standard is to set to 1.75 volts per cell or 10.5 volts.

There are a couple things going on and one is resistance in the system creating a voltage drop.

If the wire size is too small for the loaded length you will have a drop across the wire.

There are calculators for this and the requiremrnts are usually larger than you think.

A 10 ga wire is rated for 30 amps but 30 amps require larger wire for diffetent length to insure correct voltage.

12 volts is real low so it matters more.

Also batteries can have slight delay in high loads called coupe de fouit...crack the whip ...kind of like a turbo delay where the voltage dips then comes up a bit.

Setting LVD too high can cause issues like this.