How to check how much charge a battery will hold

[bbg]

How do you do it? How do you know if the batteries are excellent or ****? I have a feeling mine are shot, but I'd like to be able to measure it somehow.
Also - is there any way of measuring flow in / out on an ongoing basis, or any way of knowing how much juice the batteries currently hold at any given time?

 

[Observer]

How do you do it? How do you know if the batteries are excellent or ****? I have a feeling mine are shot, but I'd like to be able to measure it somehow.
Also - is there any way of measuring flow in / out on an ongoing basis, or any way of knowing how much juice the batteries currently hold at any given time?

1. Specific gravity test is most reliable but you can also get fair indication from battery voltage. You should get 12.74V (from memory) after all load/charge disconnected and batteries left to rest for ~6 hours.

2. Amp hour meter is one way but they have drawbacks. I have just fitted Smartgauge and really pleased with it so far. I had to replace 6x 165Ah this year (now 4 x 230Ah) which was major expense and I wanted a sure way of knowing the battery condition at any given time.

 

[William_H]

Battery capacity

A battery will be defunct when it will not start your engine reliably so capacity to provide high current has failed or the capacity to supply low current for long periods has become unacceptable.

To test for storage capacity you need to charge the battery until you reckon it is fully charged. Leave it for a few hours then attach a light globe or bank of lights of known wattage or measure total current.
Leave these lights on the battery and monitor the battery voltage. It should start well opver 12.5 volts but will reduce after hopefully a number of hours to 11v at which time you can say the battery is flat.
So typically for a 100AH battery a lamp load of 5 or 10 amps will be suitable. ie 60 watts of light globes or even an old headlamp from a car.
The number of hours the lamp can draw the near rated current before the battery drops in voltage will give you an idea of its actual capacity. If you don't know the capacity of the battery don't despair. A typical small car battery might be 30AH typical boat battery might be 80 or 100AH and will be pretty heavy to lift out of the boat battery box. Much more capacity usually ends up as 2x6v batteries just to make the weight more manageable.
Anyway what you actually get out of the battery is what you need to know. So you can compare it with your needs. A new 100AH battery might nearly give 100AH at 5 amps ie 20 hours. Higher current might give less AH. If you have a 100AH battery that is old and you get 50AH out you are doing well. A bad battery will be more like 10AH and possibly won't supply your needs.

All this is for a specific test. However if you camp on the boat and find that from what you thought was a charged battery your lights go dim after a few hours. You need an ampmeter to check the typical current drain. If drain is small like 5 amps and lights go dim after a few hours your battery is dead.
This providing you had a fully charged battery at the start. ie the gen was bringing voltage up to 14v on charge for many hours and also providing there is no hidden current sucking device that is flattening your batteries. ie bilge pump radio memory or other leakage. Or even a dead battery in paralell.
I hope this will help. You don't need a lot of accuracy olewill

 

[alan]

Battery SOC

Have a look at the "Voltwatch" website (if it still active). There is a very good description of how to perform a test .... much like Old Wills suggestion but it gives step by step instructions. This test gives a good indivation of the SOC/capacity of the batteries.

Alan.

Edit: Site still there: http://www.attfield.dircon.co.uk/info-indx.html#Testing

 

[Refueler]

Will H ... I have to comment that taking a new 100 A/hr battery for 100 A/hr is damaging to it and it's usually recc'd not to discharge a battery more than 50% of its rated capacity.

 

[bbg]

That was my thought - I don't want to damage a good battery (if it still is good). And, although Will says "you don't need a lot of accuracy", I actually do want pretty good accuracy. I have no inboard, so engine starting is by hand. My only way of charging is solar.
I have 2 x 100 or 110 amp-hour batteries. When I was on the boat a couple of weeks ago, one died (low voltage message on instruments) after what I thought was a fairly short time - a few hours. The only conusmers were normal electronics (instruments, GPS, radio) and hydraulic autopilot. So I switched to the other battery. Same thing happened to it the next day.
What I really want to know is whether the consumption - particularly of the autopilot - is so high that it flattens good batteries quickly (in which case I will need to find a way to reduce consumption and / or add charging capacity), or whether the batteries are just poor and need replacing.
I will probably have to add charging capacity at some point, but unless the batteries are decent the additional charging capacity won't be terribly useful.

 

[alan]

Battery Capacity

Just for your info: my autopilot uses about 4.5 amps when "working" (i.e. driving wheel) in a normal seaway and about 1.5 to 2 amps when "on" but at rest. The two biggest consumers on my boat (by a long way) are fridge (4 - 6 amps) and autopilot (3 amps average).

Alan.

 

[bbg]

Thanks, that's interesting.
First, is your autopilot electric or hydraulic? I have heard rumours that my particular brand of autopilot (NKE hydraulic) is a particularly heavy consumer.
Is there a way to check consumption of these users (or of all users) in "real time"? Is there an instrument that would show net flow in and out of the batteries in real time?

 

[boatmike]

Thanks, that's interesting.
Is there a way to check consumption of these users (or of all users) in "real time"? Is there an instrument that would show net flow in and out of the batteries in real time?

It's called an Ammeter.

 

[alan]

Battery Capacity

BBG: Have a look at the BEP marine website (or Merlin) they (and others) do a good volt/amp meter which also calculates capacity used, capacity remaining. I have one but to be honest I usually use just the amps and volts functions; you can see which device draws a lot of amps, and you can see what state the battery is at by reading the volts.

BTW my autopilot is hydraulic (Raymarine)
Alan.

 

[bbg]

Something like this NASA one?

http://www.nasamarine.com/proddetail.php?prod=BM1Compact

 

[jason -and the arguenauts]

How do you do it? How do you know if the batteries are excellent or ****? I have a feeling mine are shot, but I'd like to be able to measure it somehow.

If the rested frully charged battery is 12.7 volts then a half charged one (which is as low as you should go) is 12.2 volts

So what you do is to take a known load like an old style car headlight bulb - say 40 watts - and put it across the terminals checking the voltage on a regular basis. Stop when you get to 12.2 v. and work out the time taken in hours - that is T

Then the capacity is 2 x 40/12.45 x T in amp hours. OK its rough and ready but easy to do.

Dont forget that a battery can go duff in several ways - you can have a battery with an almost full capacity that self discharges cos of the crud in the bottom in a couple of hours. You can have one with full capacity that goes high resitance thats even less use.

Just done this exercise on my 2 110 AH batteries. One was 20AH and the other was 35AH.

 

[alan]

Battery Charge

A couple of points about Jason's method which is a good rough and ready check:

(1.) I would expect a fully charged (good) battery after it has been on load for a few minutes to be 12.8v

(2.) You should read the battery volts during the test when the bettrey is off load and been resting for a couple of hours, but in practice I have found resting it for ten minutes gives you a good enough indication. If you take a reading on load it will give you results thta make the battery look much worse than it really is. I have seen 2 or 3 tenths of a volt difference between on-load and rested votages: this would be about 20% to 30% error.

Alan.

 

[blackbeard]

This might, or might not, make you feel better ...
I attended a Boat Show lecturelet by Sea Start in which it was suggested that batteries be replaced after 4 years (based on call-outs due to duff batteries. I imagine this applied mainly to engine start batteries).
This seems rather disappointing compared to the 5 years or more which car batteries seem to last.
I've had batteries fail after only 2 or 3 years but I think they may have been allowed to go flat while still in the shop, or maybe had been in storage for a year or two before being sold.

 

[Plevier]

All the posts in this thread regarding discharge testing are a bit over simplistic with regard to accuracy.

Battery capacity is not an absolute number. It depends on temperature (higher=bigger), time to discharge (longer=bigger) and end of discharge voltage (lower=bigger). There are many different standards and your first problem is knowing to which one your battery capacity is stated.

I'm not up to date but from memory of working in the industry years ago the UK industrial standard was 10 hours to 1.8volts per cell at 20 deg C, the US standard was 8 hours to 1.75vpc at 25 deg C, and to make the numbers look bigger car batteries were often rated at 20 hours to 1.7vpc. The difference between these ratings can be 20% or more. As the normal end of life definition is 80% capacity, unless you are certain of the rating basis of your battery, you can't assess it with any accuracy.

Add to this that for an accurate test you need a constant current discharge. If you use (e.g.) a light bulb, it isn't. The current will decrease as the voltage drops so for accuracy you would have to measure it frequently and calculate the ampere hours from the curve.

You'll still only be accurate if the load you have chosen takes the correct standard time to reach the end of discharge voltage. If it takes less it will understate the capacity (e.g. if you double the 8 hour current, the duration will drop not to 4 hours but more like 3 hours), if it takes more it will overstate the capacity. The correction is non-linear and different for every battery design.

And of course you have to control the temperature accurately throughout the discharge.

Having said all that, a simple test like this will warn you if the battery is really snookered, but don't kid yourself you can measure with any accuracy.

You won't do the battery any harm by discharging it 100% of its nominal capacity once in a while, not going below about 10.5V for a 12V battery, but it is important to recharge it immediately. However for regular discharging the depth of discharge should be kept to 50% or less. Bear in mind too that if the battery gets less than about 24 hrs on charge between discharges with a 3 step charger, or 3 days with a single rate constant voltage charger, it won't be recovering full charge anyway.

 

[old_salt]

Has this point been missed Quote:-
That was my thought - I don't want to damage a good battery (if it still is good). And, although Will says "you don't need a lot of accuracy", I actually do want pretty good accuracy. I have no inboard, so engine starting is by hand. My only way of charging is solar.

Will a Solar charging system bring 2x110ah battery's up to anywhere near a full charge.

 

[bbg]

Good question. I don't know. Currently I use my boat primarily on weekends, and leave it on the cradle between uses. It might go several weeks between use, so I would have thought the battery should get to full charge, but perhaps not. I think the panel is about a 35 watt panel.

I will be doing some singlehanded overnight trips (eventually multi-night) and before I do I'd like to know whether to expect my batteries to stop driving the pilot at 2 am on the first night - and if so, why. If the batteries are fine I need another source of power (probably a fuel cell). If not I need to replace them and figure out how long I can expect them to last without adding that extra power source.

 

[old_salt]

With out doing the maths I would think your solar 35 watt panel would struggle to anywhere near fully charge the batteries,
A tow on a line charger may work or one of the small generators would give you peace of mind it sounds like you have an outboard so pet/oil mix is something you are used to.
Another alternative is an outboard with a 12v charge/output.

 

[markdj]

There are only a few true battery testers that give you the Ampere Hour capacity and they tend to be quite expensive.

Measuring the Specific gravity in each cell will tell you that each cell is working properly but will do nothing to tell you if the battery has an capacity - I have seen batteries 2 years old which were used ALOT and a 330Ah set only had 15Ah capacity due to bad maintenance and discharging too deep. The other way of measuring is with a constant current load discharging from 12.8 to 12.2 and measuring the time it takes.

Fully charged and after sitting for 6 hours a battery will read somewhere around 12.8 or higher. For leisure use, discharge to no more than to 12.2volts which is about 50%.

There is loads of info at batteryfaq.org

For measuring your capacity, the Sterling or BEP meters can show Ah remaining, Amps and volts; the Sterling one can read 3 different Amps ,e.g. charger output, alternator output port and starboard.