How low for agm batteries to go when UNDER LOAD?

[davethedog]

Hi all and a relatively some question,

We have a bank of 12v AGM batteries that serve the domestic and nav side etc as well as the autopilot. If say they are all drawing 15 to 20 amps (autopilot use at night with plotters, radar, fridge etc all on) how low should I allow the battery voltage to get to when under this load before looking to either turn off items or charge the batteries?

System voltage is 12v.

Thanks

DTD

 

[ChromeDome]

My Deep Cycle 100Ah AGM states

Min. quiescent voltage: 12,3 V

which doesn't say a lot about 'under load'. Depending on the actual load it may be close to the same - or far off.

12,3 V would be approx. 65% of full capacity. Me thinks.

 

[RupertW]

Depends on the amount of load but I’d be relaxed with 11.5V under heavy load provided I knew there was enough storage left to bring it back to 12.2V or more once it had rested for 15 minutes. Judging that is hard without knowing what state it was in before the heavy load kicked it, hence a good battery monitor to give a decent guess at SOC.

 

[ChromeDome]

Doesn't some of your electronics give an alarm or turn off down by themselves when the voltage drops below a certain threshold?

My fridges and heaters will shut down, nav aids will sound an alarm. On the mobile fridge the threshold can be adjusted.

Btw the fridge will keep cold pretty long when turned off if you leave it closed (as much as possible)

 

[BabaYaga]

Any meaningful answer will have to take into account the capacity (AHs) of the battery bank, which the OP has not stated.
One starting point might be to apply the loads mentioned when the bank is known to be fully charged and see what voltage reading that gives.

 

[ChromeDome]

...and to get a true understanding of the combined, actual, amps drawn...

 

[Bouba]

I use one of these to test the health of my batteries…I did this test yesterday on my bow thruster batteries



the 552 amp hour is good because the battery is only rated at 450

 

[andsarkit]

Any meaningful answer will have to take into account the capacity (AHs) of the battery bank, which the OP has not stated.
One starting point might be to apply the loads mentioned when the bank is known to be fully charged and see what voltage reading that gives.

This is exactly the point. The voltage drop of the battery under load is due to the internal resistance of the battery. A huge battery bank will have a very low internal resistance and therefore a low voltage drop for a given load compared to a single small battery.
The voltage reading under load is almost meaningless unless you have done some sort of calibration for your particular setup.
Either manually counting current and time or using a battery monitor is the only way to measure the remaining capacity.
(I think the Smartguage battery monitor does do something clever with voltage and current and learns the characteristics of the battery.)

 

[VicS]

Hi all and a relatively some question,

We have a bank of 12v AGM batteries that serve the domestic and nav side etc as well as the autopilot. If say they are all drawing 15 to 20 amps (autopilot use at night with plotters, radar, fridge etc all on) how low should I allow the battery voltage to get to when under this load before looking to either turn off items or charge the batteries?

System voltage is 12v.
Thanks
DTD

Starting with a fully charged battery, apply the load you mention, wait a few seconds for the system to stabilise and measure the battery volts
Deduct 0.5 volt and aim not to go below that figure, for that particular load..
Repeat the process for other loads.

 

[davethedog]

We have a decent battery monitor and shunt (mastervolt and easy view 5 display) so can read the volta etc easy enough.

Battery bank is 525ah (5 x 105ah agm)

The main aim of the question is to try to understand at what point we should look to start recharging the batteries on a night passage with the autopilot running etc.

Happy with the resting voltage part and when to be concerned but not 100% sure about the voltage under load side.

 

[BabaYaga]

We have a decent battery monitor and shunt (mastervolt and easy view 5 display) so can read the volta etc easy enough.

Battery bank is 525ah (5 x 105ah agm)

The main aim of the question is to try to understand at what point we should look to start recharging the batteries on a night passage with the autopilot running etc.

Happy with the resting voltage part and when to be concerned but not 100% sure about the voltage under load side.

Presumably your battery monitor will give some indication of state of charge?
I would have thought that figure would be a better deciding factor for when to recharge, rather than a voltage reading, which may vary depending on for instance the fridge compressor running or not, how hard the autopilot is working etc.

 

[Sandy]

We have a decent battery monitor and shunt (mastervolt and easy view 5 display) so can read the volta etc easy enough.

Battery bank is 525ah (5 x 105ah agm)

The main aim of the question is to try to understand at what point we should look to start recharging the batteries on a night passage with the autopilot running etc.

Happy with the resting voltage part and when to be concerned but not 100% sure about the voltage under load side.

You will know the capacity and usage. Just a matter of working out when you are 'uncomfortable' with level of usable energy in the house bank.

I sized my battery bank to last 30 hours with everything on before hitting 50% full. 525 AH looks a big bank, but you don't state your power needs.