Batteries: What is considered "low".

[smac999]

Originally Posted by ggray

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.

I would demand new batteries as part of the sale. Batteries sitting at 8v will damage them. And who knows how they have been treated before if they are allowed to do that.

[ggray]

You are right, I didn't buy it!

For other reasons.

[thomm225]

Originally Posted by smac999

I would demand new batteries as part of the sale. Batteries sitting at 8v will damage them. And who knows how they have been treated before if they are allowed to do that.

But if he liked the boat he can get new batteries for around $80 each.

Not a big deal at all

When I was a teenager I would use my car battery on my boat.

Drive to the ramp.

Put boat in.

Park.

Move battery to boat etc etc

Folks on CF love to make a big deal out of batteries, anchors, solar, etc,,,,,,

[GILow]

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?

As others have (mostly) agreed, battery voltage isn’t a great test of battery state of charge when there is a draw on the battery.

But we need a bit more information to make a meaningful suggestion. The first would be what is your total battery capacity? Battery brand might also help as there are sometime minor voltage variations between manufacturers.

However, what I believe you REALLY need to do is measure your actual useable battery capacity before you do this trip. There must be hundreds of threads here from people who have set off on their first significant passage only to find that the battery bank that seemed totally fine when they were returning to the dock to recharge each night was actually almost dead and had perhaps as little as 10% of its theoretical capacity. (Myself included.)

The process for measuring capacity is pretty simple and does not require a lot of expensive tools, just a multimeter and some old fashioned car headlight globes or some other means of putting a moderate, continuous load on the batteries.

Well worth doing.

And then buy a decent battery gauge, they are a terrific addition to any cruising boat, and not all that expensive.

[coopec43]

Originally Posted by ItDepends

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

Thank you for that! I was just following the installation instructions so I suppose they didn't want to get too technical.

(I suppose your comment made you feel good?)

[GreenWave]

Originally Posted by GILow

As others have (mostly) agreed, battery voltage isn’t a great test of battery state of charge when there is a draw on the battery.

But we need a bit more information to make a meaningful suggestion. The first would be what is your total battery capacity? Battery brand might also help as there are sometime minor voltage variations between manufacturers.

However, what I believe you REALLY need to do is measure your actual useable battery capacity before you do this trip. There must be hundreds of threads here from people who have set off on their first significant passage only to find that the battery bank that seemed totally fine when they were returning to the dock to recharge each night was actually almost dead and had perhaps as little as 10% of its theoretical capacity. (Myself included.)

The process for measuring capacity is pretty simple and does not require a lot of expensive tools, just a multimeter and some old fashioned car headlight globes or some other means of putting a moderate, continuous load on the batteries.

Well worth doing.

And then buy a decent battery gauge, they are a terrific addition to any cruising boat, and not all that expensive.

While a straight up measure of voltage with voltmeter is only a partial story, we know that any 12v battery that has a voltage below 11 volts is in trouble and below 10 is toast. So it is a starting point to put a meter on them.

[GILow]

Originally Posted by GreenWave

While a straight up measure of voltage with voltmeter is only a partial story, we know that any 12v battery that has a voltage below 11 volts is in trouble and below 10 is toast. So it is a starting point to put a meter on them.

Sorry, but no, it depends on the load, relative to the battery bank size. I can pull my bank of T105s down to 11.9 by turning on the coffee machine, but they are healthy as can be.

Ok, 10 volts is a sure sign of trouble, but the problem is a reading of say, 12.5 means nothing in itself to indicate the true health of the battery. What if there is some solar connected, what if the engine is running, what if some lights are on… etc, etc.

Voltage is only really of value to people who know their boat and their battery bank well enough to make a meaningful extrapolation. The PO has already said they don’t have a battery monitor, and this sounds like they have not done any big trips yet, so there simply isn’t likely to be any kind of knowledge of the bank to meaningfully extrapolate from voltage.

There’s no substitute for a good old fashioned capacity test, followed up with a half decent energy budget.

Voltmeters, in this situation, will be misleading at best and downright wrong at worst.

[GreenWave]

Was referring to unloaded voltage. If I put an AA battery on my voltmeter and it is 1.3 or less it is done! For a 12 v cell anything below 12 is questionable when unloaded.

[coopec43]

Originally Posted by GreenWave

While a straight up measure of voltage with voltmeter is only a partial story, we know that any 12v battery that has a voltage below 11 volts is in trouble and below 10 is toast. So it is a starting point to put a meter on them.

"below 10 is toast"

Is that necessarily true? (Probably is). But:


Can solar battery be revived?
"It can take several days to a couple weeks to revive a dead battery, depending on how bad the battery is, the size, and how often you cycle(charge and discharge) the battery with BLS attached".

AND

Reviving a Dead Battery
The Battery Life Saver electronic device can be used to revive a dead battery that has accumulated too much sulfate build up or has been allowed to sit unused for a long period of time.
However, it will not work on a battery that has a shorted or open cell. See the "How to Test for a Short or Open Cell" page for details on shorted cells and how to tell if your battery has them.
This procedure works for a car-auto, deep cycle, marine and all other lead acid battery applications. It will also work with AGM and GEL batteries.

https://thesolarstore.com/how-to-rev...ml?newsPath=15

[coopec43]

My Dad's favorite trick!

How to Desulfate a Battery With Epsom Salts

"When car batteries aren't used for a long period of time, they lose their ability to hold a charge. Over time, the lead from the batteries' internal plates combine with electrolytes to create lead sulfate. When this happens, you must completely desulfate the battery so that it again holds a charge. Desulfation is also known as battery "reconditioning." Because batteries contain toxic chemicals, such as lead acid, you must take safety precautions. Always wear goggles and gloves when performing this procedure.

https://itstillruns.com/desulfate-ba...-10076192.html

[GILow]

Originally Posted by GreenWave

Was referring to unloaded voltage. If I put an AA battery on my voltmeter and it is 1.3 or less it is done! For a 12 v cell anything below 12 is questionable when unloaded.

I think the OP would know by now if their batteries were that dead.

What they were asking about, and really do need to know, is if their battery capacity is going to be adequate.

The only way to do that with any degree of accuracy is a capacity test and an energy budget.

A voltmeter will tell them next to nothing, and, as I will keep repeating, it will, most probably, be misleading.

[thesaltytar]

Agreed. Another problem I didn't see mentioned (maybe I missed it) is surface charge. A resting battery voltage is only part of the picture of a battery's health and can mislead.

I have always used a battery load tester for AGM or gel. If I recall correctly it measures the current across a known and applied load.

Lead acid I have always tested specific gravity with a hydrometer or refractometer, cell by cell.

[Adelie]

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?

I would consider being at 50% or less to be low.
MaineSail has SoC vs Voltage chart for no load and under load conditions.
https://marinehowto.com/under-load-b...oltage-vs-soc/

Looks like 12.1-12.2v is about 50% when under moderate load.

I’ve seen charts with multiple curves for different C loadings.

If you have a 200aahr bank you could probably start an inboard no problem. I would discharge the battery to 60% at the dock and test whether you can start the engine. Make sure all other loads are off for a few minutes.

I would go get a Lithium battery pack or a super capacitor jump pack as a backup.

AGMs are mediocre for house loads, cycle life is compromised is they aren’t recharged fully every day and if the are discharged to 60% or lower they need to be charged at 0.2C minimum (20A for a 100Ahr bank) 0.3-0.4 is much better.

[Dockhead]

Originally Posted by Adelie

I would consider being at 50% or less to be low.
MaineSail has SoC vs Voltage chart for no load and under load conditions.
https://marinehowto.com/under-load-b...oltage-vs-soc/

Looks like 12.1-12.2v is about 50% when under moderate load.

I’ve seen charts with multiple curves for different C loadings.

. . .

An acquaintance of mine who is a battery engineer in the military-industrial complex and happens to also be an RV-er, told me that MaineSail's test linked here is wrong, that there was some mistake in the methodology (probably the resting voltage curve is wrong he said). He told me that it is impossible on the basis of fundamental physics, for the voltage under load to be HIGHER than the resting voltage at the same SOC -- that any loads, however big or small, must needs REDUCE voltage, and that this is true with all battery chemistries. Your "charts with multiple C loading curves" will confirm this. More C, and lower voltage, every single time. It cannot be otherwise, so MaineSail made some kind of mistake somewhere.

MaineSail's bottom line is fine -- charge at 12.1v (24.2 for us 24 volters). Sure. But note also that you are not indeed "murdering your batteries", by occasionally dipping 0.1v or 0.2v below your target minimum. There is no cliff the batteries fall off of. Cycle life is reduced smoothly with greater depth of discharge, all the way down to 90% or whatever; there is no magic to 50%. For this, see the cycle life vs. DOD charts in any battery manufacturer site -- you will see smooth curves.

By the same token, it's beneficial to charge BEFORE 50% DOD whenever you can -- the cost in cycle life of a 40% DOD cycle is less than a 50% DOD cycle.

And in any case, more harmful than an occasional 60% DOD cycle is partial SOC cycles where you don't get the batter back to 100% before you start discharging it again. And this is inevitable off grid unless you have a large solar installation, big enough not only to supply your needs but with enough power left over for a long finishing charge every day.

Bottom line -- watching voltage is a perfectly adequate way of managing a lead-acid battery bank. Charge early, charge often, charge a long time -- is the right motto for lead acid batteries. A lot of solar is the ideal environment for lead acid batteries, and a heavy duty generator which is run a lot is somewhat ok. I have the heavy duty generator, and I put a lot of hours on it when I'm off grid. And even so don't get the 100% finishing charge on the batteries every day.

LiFePo4 is of course fundamentally better because we don't have to worry about getting the battery up to 100% and it has such a great cycle life that we don't care about an 80% DOD. Just treat it like a fuel tank. What a dream.

[ovidiunelu]

Get one of those and get peace of mind https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1