Battery load test

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alanporter

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I note from previous posts that 12 volt batteries are better load tested rather than just reading open circuit voltage. I was thinking of testing my batteries by connecting them to a 1000watt invertor into which is plugged in a 800 watt microwave oven.
What thinketh the experts about this ?
 
It rather depends what you mean by "test". If you simply want to see whether your battery will drive a big load then your suggestion might be reasonable (if you are sure that your 1000W invertor can actually drive your 800W microwave? There might be issues re. inrush current). If OTOH you want to be a bit more scientific and put numbers to it then I would recommend something more conventional using a resistive load and a meter.
 
[ QUOTE ]
if you are sure that your 1000W invertor can actually drive your 800W microwave?

[/ QUOTE ] Indeed. The consumption of our 800w microwave is 1420w.

Are the electronics of a microwave robust enough to withstand the output from an ordinary inverter or do they need one with a pure sine wave output.


This is all irrelevant any way just put a 1kw electric heater on as load.
 
Battery Load Tester ....... google and you'll find various companies do meters that impose a load and give read-outs for 25 quid and up ..... Draper, Towsure, etc.
 
i agree with everything said so far.

just an opinion tho'....

if it's a start battery you will need maximum amps at cranking speed. one way of testing the battery for a fault is to watch the electolyte to see if it bubbles under load (that's what the load test is for.... the cherry red thing)... if a cell does bubble then there's a good indication that the cell is faulty.... i've repaierd this in the past by washing the muck out of the battery and refilling with filtered (clean) electrolyte... probably not a permanent solution, but it worked for a while.

i'm always carefull when testing batteries this way as the gas they are giving off is explosive, so if there was a spark it's easy to blow the tops off the battery and resultant mess isn't funny.... quite apart from the obvious danger.

you need to find a battery company to test yours... it's better than trying to watch the cells with someone cranking the engines... although it's one way of doing a quick test.

don't ever be tempted to short the battery with jump leads, although it will load the cells, it will almost certainly warp the plates... then kill it.

don't know if this help... but it's safer to take the battery for test rather than try to simulate a load yourself.... even so it wouldn't stop me from doing the quick test.

steve.
 
A FEW POSTS BACK..

RE.. Marine engineers with short circuit testers..

A warm glow= good battery
No glow= Change Battery

A Reputable marine engineer I know has two teaters, one for customers he wants to sell new batteries to, and another to customers he thinks he can not get away with it.

Who can you trust??
 
Load tests are normally only done on standby batteries maintained on full charge (so not cycled) or those cycled on remote sites where they cannot be readily observed (eg solar powered light houses, radio transmission/relay, etc).

For batteries in normal cycled use, such as on a boat, their condition, as would be tested by a load test, is always obvious due to the capacity experienced when cycled in use and other signs such as increased water use. Their normal cycling is a load test in itself and loss of capacity is obvious from reduced run time.

If the boat is new to you so that previous experience of its battery cycles is not there to refer to then I just make an estimate of usage over normal discharge cycle and compare to the rated capacity of the battery bank - it is usually obvious if their useful life is coming to an end.

If just interested in a test as to whether they will hold voltage under a load then if a cranking battery just crank the engine with the fuel stopped (but not long enuff to flood any water lift muffler and hence the engine). But if it starts the engine ok when cranking normally then it must be fine. Again for service batteries if they are being used (rather than in standby service) then loss of capacity is pretty obvious but any big load for a couple of minutes will show if they hold voltage or not (obviously in both those cases the voltage will drop while actually under the load).

I, for myself, would never load test my cycled batteries as any deep cycle is just another step on the road to their ultimate demise and expensive replacement, and doesn't tell me anything that I did not know already.

John
 
I tend to agree with John a starter motor is a very good load test of a battery. Any hesitation or slower than usual cranking means it is getting old. If the battery is intended for deep cycle storage it should still be OK for occasional engine starting (small engine).
If you don't want to use the engine starter then just monitoring of voltage with all the usual loads of lights inverters winches radios etc should also indicate impending failure.
I think the trick with batteries is to be familiar with performance and volt drop under a load when the batteries are good such that any demise becomes obvious. Cos they surely will die of old age.
Hence if you have any doubts buy a new one rather than spend money on testers etc. olewill
 
Sorry to disagree with you on that but IMO that doesn't really apply to liveaboards, marina boats and boats with domestic battery banks that go for months without any seriously discharge or heavy load. These battery testers don't even begin to 'cycle' the battery, they are just a short 10 second load test to check that all cells are low resistance and functioning. It would be appropriate to carry out a test monthly if you wanted to, or you could wait until shortly before the cruising season, leaving time to order and fit new batteries (most liveaboards I know don't use any old batteries, they choose quite carefully from what is available locally and it can take time.
 
Hows about just cycling the batteries through a moderate discharge with the shore power supplied charger turned off - which is what I do with no problem.

John
 
[ QUOTE ]
Hows about just cycling the batteries through a moderate discharge with the shore power supplied charger turned off - which is what I do with no problem.

[/ QUOTE ] I know quite a few yachtsmen who believe that with batteries "use it or lose it" applies, both for main batteries and their laptops. This simply isn't true; all batteries have quoted maximum charge/discharge cycles before they can only hold 80% (or some other percentage) of the initial design capacity. So cycling, per se, is not a good idea. As for putting the battery on discharge, yes, that is what I am talking about and that is what a tester does. But a tester draws, say, 300A for 10 seconds i.e. about 0.8 Ah.

It is a completely different thing to a moderate prolonged test and tests for the resistance of the cells, which a moderate test cannot. That is why these testers are made, after all!
 
I am not proposing cycling from a "use it or lose it point of view" - I am sure I have posted elsewhere about the fallacy of that and allude earlier in this thread to avoiding deep cycling for the sake of a load test.

But for the case you raised of a boat that sits a long time in the marina with batteries on charge (so similar to the standby battery case I mentioned before) just go through one cycle on normal type loads (to estimated 30% discharge say based on loads on the boat - I have a pretty good idea of our usual consumption under various circumstances) as a "load" test. Is what I do if I have indications the batteries are losing capacity (and those indications are always obvious before the time loss of capacity reaches the point of raising concern), is not tremendously accurate as one usually estimates the AHrs drawn if the cycle has different loads, being in the marina rather than at sea/anchored, to what one is familiar with the run time for (or use a monitor if one exists) but certainly works sufficiently to make a good enough judgement about the batteries.

I would do that in preference to a heavy load such as the original poster proposes for the main reason that a heavy load will temporarily reduce the capacity of the battery during the discharge to it and so may give a false impression.

I mentioned before monitoring volt drop under a short load if internal resistance is what is wanted to check, but again there will be other in service indications of abnormal voltage drop under load if such is a problem.

Of course, if people want to buy gear to test their batteries that is just fine by me and they are welcome to. I'm just pointing out that there is no real need to (and I wouldn't use my own money to do so) and I guess that is the end of the matter.

John
 
Some years ago I bought a "Voltwatch" from this company. One of the useful bits of info from the website was how to test your battery (bank) capacity safely. The link below should take you to the relevant part of the website. The procedure means that you have to spend 12 hours or so with a voltmeter and measure the battery volts every hour (or whatever period you chose) or so. I bunged the results into a spreadsheet and made a pretty graph, so that when I checked the next time I could quickly see any dofferences.
I found the logic behind this method of testing better than what has been suggested in other replies.

web page

Alan.
 
That's an interesting idea that I haven't come across before. It is a bit complicated, though? I agree with Ship's Cat that in general one gets a very good idea of one's batteries from experience provided you have a good monitoring system (e.g. Link or at least a good DVM and ammeter). Periodic high load checks are also important especially for batteries that don't see cranking service.
 
Our only disagreement seems to be whether a high-load short duration test can be useful. If I can buy a tester for £25 and it allows me to judge more accurately when to change my batteries then I will be well pleased.

Actually, I have a different problem. I do not have a cranking battery but use my three semi-traction batteries in parallel. So while they are individually rated to crank, they should only ever see A/3 Amps when I crank. Actually, the bowthruster is my biggest load and in one sense would suffice but it is not repeatable so I cannot judge how the batteries are deteriorating over time simply by using the bowthruster (the load increases as the prop gets fouled which it invariably does over the winter).
 
I don't think we disagree on a short duration test being useful, just that I am pointing out that you don't need to buy any equipment to do it accurately enough to know whether there is a problem or not.

For a short term load to check voltage drop (internal resistance) I just put a big load on the inverter eg toaster, heater on low, etc (our vacuum cleaner is good, it draws 100 amps, as is an adjustable output hot air gun /forums/images/graemlins/smile.gif). I recognise that on some boats it may not be easy to sink such loads with normal consumers. But again, I am familiar enough on my own boat with the normal voltage drops on the DC panel voltmeter when lesser consumers come on line (eg freezer which draws around 35-40 A off 520 AHr bank) to recognise any developing problem.

Regarding the proposed voltage monitoring method, it has obvious difficulties with large banks (one assumes the original poster has such as refers to a 1kW invertor) associated with sinking a long drawdown at around C20 apart from the inconvenience of having a dead ship for the duration of it (and the long draw down is necessary to get significant voltage drops). For cranking batteries and smallish single cycling batteries it is always easy to judge their condition from the observation methods - including the short duration heavy load. Unless, of course, one wants to try and be exact in the measurement of capacity but I am not sure what practical benefit that is for most users.

John
 
I am a Marine Electrician (yachtelectricalservices.co.uk) and I certainly do not have 2 testers. Beware making a load (drop) test on a battery, sparks can fly. Certainly don't do it near a petrol engine. You can get a good indication by cranking your engine and measuring the battery voltage whilst cranking. web page
 
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