Testing Battery drain

z1ppy

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9 Mar 2008
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New Forest
Morning All,

looking for some advise from the technical collective....

How can you test the drain on a battery or batteries (when connected together) on a boat?

We have 2 batteries on board with a 1 / 2 / both / off / isolation. I suspect there is a significant draw on the batteries either when isolated or when running...

I am not technically minded so need to try and understand how to measure the draw, i assume that if we start with everything off then switch things on we can find out where the draw comes from and thus what is flattening the batteries?

From there we can also establish what power is being returned to the battery via the alternator....

any help greatly appreciated as i am scratching my head here.. (please try and be as simple as possible as i am dumb!)
 

maby

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12 Jun 2009
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First you need to distinguish between an unwanted current drain in the equipment of the boat and one or both batteries having failed. If you can isolate them as you suggest, then get them well charged either via shore power or by running the engine, then promptly isolate them and leave them disconnected for several days. Healthy batteries should not lose much charge over a significant period, so when you reconnect them you should still have plenty of power to run lights and instruments. Does the boat have a volt meter fitted? If not, you need to get a cheap multimeter - Maplins will sell you something perfectly useable for under a tenner - and check the voltage at the battery posts. A battery on charge will have a voltage of over 14v as it approaches full charge. When you disconnect the charge (turn off shore power or stop the engine), this will quickly drop to about 13.5v and then drift down to about 12.8 or thereabouts. If the battery is isolated and in good condition, it should be able to hold a voltage around 12.5v for many days, if not weeks.

If you find that the voltage of isolated batteries is drifting down quite rapidly, then one or both of them has probably failed. It is worth just checking that they really are being isolated by the big switch - get them fully charged again, then actually disconnect the cables from the battery posts and keep an eye on the voltage. If you have multiple batteries connected in parallel to provide the bank capacity, then the failure of any one of them can pull down the rest with it. We have a domestic bank made up of four 120Ah batteries which started to lose capacity - by charging them fully, then isolating each of them and monitoring the voltage, we were able to identify two of them that had failed while the other two were fine - we disconnected the two bad batteries and our capacity shot up.

If you check them as above and they all seem fine, then it's time to try looking for a rogue load that is puling them down - that really comes down to a question of disconnecting things in turn till you find the culprit. This will be a lot easier if you have an ammeter to measure the current being drawn. If you don't have one fitted in the boat, you can use a multimeter temporarily connected between the boat and the battery, but you will need to buy one of the more expensive meters - equipment in boats can draw quite high current and the cheap meters will not be able to go high enough. £35 at Maplins will buy you a meter that reads up to 20A which should cover most items on a boat. Note that the engine starter motor will draw far higher, so don't attempt to start it with a meter connected into the circuit - you will almost certainly destroy it!
 

gordmac

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Lochaber
I wouldn't use a meter to check the alternator output current, it can easy be more than 20A.
 

z1ppy

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New Forest
slow progress,

batteries came home and went on charge for 4 days, back on the boat and...

Alternator is charging well

Batteries tested and showing as good. Have left them on and connected for a week of so (as I'm busy next week) and will test again.

i am going to replace the batteries as speaking what Barus at the show yesterday, the general thoughts were dodgy battery as primary cause.

will update more next weekend i guess as I'm away for most of this week coming.
 

rafiki_

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19 Jan 2009
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Stratford on Avon
T
slow progress,

batteries came home and went on charge for 4 days, back on the boat and...

Alternator is charging well

Batteries tested and showing as good. Have left them on and connected for a week of so (as I'm busy next week) and will test again.

i am going to replace the batteries as speaking what Barus at the show yesterday, the general thoughts were dodgy battery as primary cause.

will update more next weekend i guess as I'm away for most of this week coming.
Battery Megastore? Did me a great deal a week or so ago.
 

z1ppy

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New Forest
yes, although Barden batteries down here have a boat show special price if ordered before next sunday making them £20 a battery cheaper than battery megastore and i can collect
 

rafiki_

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Don't go with the on line price. Best give them a call. I got a further 30% discount on the phone. Delivery is £5, although I collected as not far out of my way.
 
Last edited:
Joined
19 Aug 2010
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Location
UK; France; Spain
Hi buddy,

Soggy lockers? Had a similar prob with RIB. I found the under seat battery locker wet with condensation or weather infiltration or both and as the the master switch was in the same environment decent battery life had no chance. Resealed the locker lid, replaced batts (which were found to be incorrectly installed, with domestic type as engine start) but master switch dried out and stayed okay. Barden supplied batts at okay price and in stock.
 

ratbag

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31 Dec 2010
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ireland
test for drain by fitting ammeter in series with the cable , if there is a load when everything is off, remove/switch off fuses until there is no current draw.
 

Freeloader

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22 Mar 2007
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Location
Home: Rutland, Boat: Poole.
If you have two batteries running through a '1/2/Both/Off' switch, then there shouldn't be a drain on both batteries when the switch is in the 'Off' position - some items, such as the bilge pump, radio memory etc, will bypass the switch and still have a small draw but these ought to be connected to only one of the batteries. In that case, the battery with the load should display a gentle discharge whilst the other battery should level out and remain around 12.8v, as stated above. Note that the wet lead acid batteries usually fitted to boats will self-discharge gradually anyway and will need topping up quite regularly regardless of any load.

Before you rush out to replace the batteries, though, you really need to determine whether they are maintaining a good voltage when isolated. It will be impossible to determine how good the batteries are if you have a spurious load on your boat - they need to be disconnected to properly monitor their voltage over a period of time. A multimeter is definitely a good start and is a cheap way to monitor your batteries. Better still is to fit a dedicated battery monitor such as a Victron Energy BMV-702 - these are also available from Barden (from personal experience I have found their customer service / backup to be excellent too).

The battery monitor will not only show you battery voltage but also the amount of current going into / out of the battery, the total amphours drawn and % state of charge, amongst other things. It will, however, only fully monitor one battery and simply display a voltage for the second battery. Really, you ought to use one battery purely for engine starting and the other for all other 'house' functions. This will leave one battery always isolated and in a full state of charge (for engine starting) and the other providing all your services and backup functions such as the bilge pump, radio memory etc. You can use your '1/2/Both/Off' switch to achieve this but a better way is to automate the process so you just have an on/off switch for the services, as this won't rely on your remembering to frequently change the battery switch position. A relatively cheap way to achieve this would be to fit a Blue Sea Systems 'Add A Battery' kit. This contains an on/off switch and an automatic voltage sensing relay, which will put all the charge to the 'engine start' battery and, as it comes back up to full voltage, link in the 'house' battery to charge that as well.

Back to the original thread.... Once you have established whether your batteries are any good, you can then test the system for abnormal loads. Either with a multimeter or a battery monitor you can test for loads with the battery switch in the 'off' position, then the 'on' position and then, one by one, by switching individual services on / off. A battery monitor will be able to test far larger currents than a multimeter's typical 10A and so you will also be able to test your battery charger's output and check the system with the engine running, which will ensure that the alternator is actually charging the batteries in the first place!

Incidentally, I have used Battery Megastore too and received some very helpful advice. For bow thruster and engine start applications, I bought Oddessy PC2150 100Ah AGM batteries - these have a massive 1150CCA rating to provide plenty of cranking power, even when not fully charged. Being AGM, they will also sit happily for months on end without significant self-discharge and so, even if you don't have shore power, they will still always be ready to go.
 
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