Batteries: What is considered "low".

[Rockinar]

Got a new auto pilot and Dometic fridge on my boat and doing a longer passage next week. Boat does not have a battery monitor. I was going to check the batteries (AGM) with a multimeter on occasion.

What would be considered "low"? I want to make sure I dont drain them to the point where it wont start. Is there a "safe" zone?

[S/V Illusion]

measuring voltage tells you very little about capacity. If you don’t already have a dedicated “start” battery, it’s time to rethink the electrical system.

[smac999]

12.0v is generally where you stop draining a battery.

But that is when it doesn’t need to start an engine.

You should not be draining a battery that needs to start a motor. You should have 2 banks.

If it does need to start one. Then Maybe stay above 12.5v.

[coopec43]

I'm not going to argue with the other guys but I asked the same question just a few days ago. I hooked up my solar panels to the house bank and checked the voltage of the batteries in the morning and they were 25.8V (24V system). Then I checked them in the afternoon and they were 25.9V)

I decided to read the manual and found the Victron MPPT solar controller doesn't charge if the battery is over 25.8V.

I then did a google on "voltage of a fully charged AGM battery" and got:

12.8 to 12.9
The sealed AGM and gel cell battery voltage (fully charged) will be slightly higher in the 12.8 to 12.9 range. If you have voltage readings in the 10.5 volt range on a charged battery, that typically indicates a shorted cell.

https://www.batterystuff.com/kb/arti...ry-basics.html

[OldManMirage]

Originally Posted by Rockinar

Got a new auto pilot and Dometic fridge on my boat and doing a longer passage next week. Boat does not have a battery monitor. I was going to check the batteries (AGM) with a multimeter on occasion.

What would be considered "low"? I want to make sure I dont drain them to the point where it wont start. Is there a "safe" zone?

We need more info. I can tell you the frig will use a good deal of power. I've been experimenting on my boat. I've got 200 watts of solar and 230 AH of battery power using two 6volt golf cart batteries. So far it managing to keep the frig running as long as I don't try to keep too cold.

But I also have a 2nd "engine" battery so I can always crank up the motor if I need to.

So for a good answer we need to know how much battery capacity you have and what do you have to keep it charged ?

[Bobby Lex]

I guess it's easy for us to spend your money....

....but honestly, for as long as I've been sailing (since the 1970's) the cardinal rule has always been to have a separate, dedicated engine starting battery. So if you are not already set up that way, you need to invest in a separate engine starting battery and a marine battery selector switch, and make sure that the engine starting battery is only used for starting the engine.

....next, you do need to invest in a battery monitor. Your battery charge status is critical information, no less so than a temperature gauge for your engine. There are two reasons for this:

1. One, is the reason you mention, you want to make sure your engine starting battery has enough juice to start your engine when needed.

2. Two, is because you do not want to discharge any of your batteries below 50% as a general rule, before recharging them. If you don't follow this rule, then you are likely to damage your batteries and shorten their useful life considerably. Having to replace your batteries every year or two will cost you more than a battery monitor.

So, there it is, I've managed to spend about $500.00 of your hard-earned dollars. But in the final analysis, that's money well-spent.

Bob

[thomm225]

Most anything will work as a battery monitor.

I've been using the PO's for the last 10 years....

Now I use that plus my Victron Display which isn't always on so still relay on the old analog battery monitor.

Many times over night my batteries used to go down to 11.8 volts until I got the golf cart 6 volt batteries now lowest I've seen is around 12.3-12.4 volts.

I don't have a starter battery. (don't have a starter motor)

[ItDepends]

Originally Posted by coopec43

I decided to read the manual and found the Victron MPPT solar controller doesn't charge if the battery is over 25.8V.

This is a very inaccurate, and simplistic paraphrase of what a Victron MPPT actually does.

[cr180]

Battery vendors publish information depicting lead acid battery state of charge vs voltage. Google Trojan Battery Maintenance, for example (12.73 - full charge, 12.5 - 80%, 12.1 - 50%, 11.81 - 30%, 11.51 - 10%). So even without a battery monitor, you can easily determine state of charge with a multimeter. A rule of thumb used by many cruisers for lead acid batteries is to try to keep from routinely discharging below 12V in order to maximize life of batteries. A second criteria for an occasional passage would be to stay above the minimum voltage needed by fridge and autopilot. You might be able to get away with 11.8ish. Above discussion limited to house bank, NOT starting battery.

[S/V Illusion]

Originally Posted by cr180

Battery vendors publish information depicting lead acid battery state of charge vs voltage. Google Trojan Battery Maintenance, for example (12.73 - full charge, 12.5 - 80%, 12.1 - 50%, 11.81 - 30%, 11.51 - 10%). So even without a battery monitor, you can easily determine state of charge with a multimeter. A rule of thumb used by many cruisers for lead acid batteries is to try to keep from routinely discharging below 12V in order to maximize life of batteries. A second criteria for an occasional passage would be to stay above the minimum voltage needed by fridge and autopilot. You might be able to get away with 11.8ish. Above discussion limited to house bank, NOT starting battery.

A fundamental omission here is that any indication of capacity by measuring voltage must be done “at rest” and any load can mislead the correlation. As stated in the OP’s question, he queried voltage levels while running equipment.

[cr180]

Originally Posted by S/V Illusion

A fundamental omission here is that any indication of capacity by measuring voltage must be done “at rest” and any load can mislead the correlation. As stated in the OP’s question, he queried voltage levels while running equipment.

True. But even under load, I’ve found the the no-load numbers to be a practical approximation of state of charge over many years of cruising.

[Dockhead]

Originally Posted by S/V Illusion

A fundamental omission here is that any indication of capacity by measuring voltage must be done “at rest” and any load can mislead the correlation. As stated in the OP’s question, he queried voltage levels while running equipment.

Using system voltage under light loads is very useful. The depth of discharge under load will be less than the DOD indicated in the open circuit chart. Read it "at least" x% remaining and you have extremely useful information. The error is all on the conservative side and it won't be so much if the loads are reasonable.

It's the opposite with amp-counting battery meters, which I consider worse than useless. The errors - which can be large when you're off grid for a while - are all on the reckless side, OVERSTATING your SOC and tempting you to damage your batteries.

Sometimes the old ways really are best. There's been good advice in this thread.

[Dockhead]

Originally Posted by cr180

True. But even under load, I’ve found the the no-load numbers to be a practical approximation of state of charge over many years of cruising.

[thomm225]

Originally Posted by Rockinar

Got a new auto pilot and Dometic fridge on my boat and doing a longer passage next week. Boat does not have a battery monitor. I was going to check the batteries (AGM) with a multimeter on occasion.

What would be considered "low"? I want to make sure I dont drain them to the point where it wont start. Is there a "safe" zone?

Just hook some longer leads to your meter and simply turn it on when you want to check the voltage.

Then back off to save the meter's battery.

You'll get used to what is normal and what isn't.

I could go as low as 11.8 volts but didn't have to worry about starting the engine.

My solar would charge it back up quickly as the Sun came up or slowly if it was cloudy.

The batteries I have now though have never gone below 12.3 volts or around 60% on my Battery Power Monitor

[ggray]

Originally Posted by cr180

Battery vendors publish information depicting lead acid battery state of charge vs voltage. Google Trojan Battery Maintenance, for example (12.73 - full charge, 12.5 - 80%, 12.1 - 50%, 11.81 - 30%, 11.51 - 10%). So even without a battery monitor, you can easily determine state of charge with a multimeter. A rule of thumb used by many cruisers for lead acid batteries is to try to keep from routinely discharging below 12V in order to maximize life of batteries. A second criteria for an occasional passage would be to stay above the minimum voltage needed by fridge and autopilot. You might be able to get away with 11.8ish. Above discussion limited to house bank, NOT starting battery.

I was looking at a used boat, 22ft outboard with two batteries, with a possible sea trial afterwards. Broker was going to connect a battery charger first, but I wanted to get a sense of the batteries before charging by checking voltage first.

I did, and found them to be 8v.

Broker said "they are 2/3 charged". Hmmm.