Are my Service Batteries any good?


Lithium Ion batteries have to have within them a sophisticated Battery Management System (BMS) which will tell you a lot of data, including number of cycles so you can gauge how long they might still live – usually at least 2000 cycles is a good life, but that figure is going up each year.

So, I’m talking about Valve Regulated Lead Acid (VRLA) batteries here – the generic term for all those big lumps of lead you’re hiding (usually) low down in your superyacht. That’s a lot of expensive weight to carry around if it’s worn out. But because they are expensive and awkward to replace, how do you know if you have good service life left in them, or whether they are an almost useless, heavy mass? On one superyacht the engineer didn’t even know where the service batteries were located it had been so long since they were tested!

There’s basically two methods I use to assess VRLA batteries condition; one is fast and easy, the second is slower but gives you valuable data.

  • 12V battery tester: A small, microprocessor based 12V battery tester can be obtained for a modest outlay and I find gives good repeatable readings on the Cold Cranking Amps (CCA) measurement of 12V automotive and SLI batteries I often find on superyachts. Make sure you can find out the manufacturer’s ‘as new’ CCA and what standard it is tested against (there are at least five: DIN, EN, IEC, SAE, CA). I use the Sealey product, see this link It’s amazing how evenly batteries in a bank wear. If the CCA is more than 20% lower than the manufacturer’s specified figure at normal room temperature, I recommend you replace the batteries soon. Obviously, this type of little tester is also very suitable to check your starter batteries.
  •  Actual discharge Ampere Hours measurement: For those large, 2V cell battery banks (as on the left in the photo), you cannot test them with method 1. There are all sorts of sophisticated set ups and special, costly equipment to measure the Ampere hours (AHr) of the whole bank, but I’ve found an easy and most reliable way to measure them is put them through an actual discharge test, as follows:-
    1. The whole battery bank must be fully charged first, so must be on the ‘float’ phase of the charge cycle (*)
    2. These are rough an ready calculations, but we want to discharge the bank over a period of 10-20 hours, meaning to take current out at the ‘C/10’ or ‘C/20’ rate, which is a relatively low rate. And we want to ensure that each cell does not drop below an equivalent of 11.0V for 12V battery, or 22V for a 24V battery i.e. does not drop below 1.83V for each 2V cell. So, say each 2V cell is nominally rated at 1200AHr (quite common, they are big and heavy at well over 60kg each), so C/10 is a 120A discharge and C/20 is a 60A discharge rate. Let’s say this is a 24V battery bank of two ‘strings’ of 2V cells in parallel (very common on superyachts, especially sailing superyachts which need extra battery power for sail controls at certain times). So, in fact the C/20 rate will be 60A per string, so 120A total from the two strings in parallel. For a 24V battery bank, that will represent a discharging power drain of – yes you guessed it! 24V x 120A = 2.88kW.
    3. Now we just need to find a load of around 3kW to take a nice easy, long slow but relatively constant drain out of the batteries. Hard? No! Almost every superyacht will have inverters on board which can deliver many kW of AC power to essential loads. We just need to plug a 2.5 – 3kW space heater on the AC power out of the inverter and there’s our nice 100+ Amps drain.
    4. This will take some organisations and agreement on boar, but make sure that heater (via the inverter) is almost the only power drain on the batteries and start to monitor and record the voltage and current every hour using a DC clamp meter on the single main DC feed from the battery bank. Start your clock with the start of discharging at that nice constant rate. Meanwhile, do something else! Make sure no other big loads can start (or stop) which will interfere with the following discharge current measurement steps.
    5. If you ensure you terminate the discharge close to that figure of 1.83V per cell then record the final elapsed time and current, then with your data points from each hour, you can  calculate the Ampere Hours, hour by hour, drawn from the battery, to the final 1.83V per cell termination point.
    6. Compare your calculated AHrs for the whole bank, with the nominal AHrs of the original ‘as new’ batteries – 2 x 1200Ahr = 2400AHr – and you have a pretty good idea of your service battery bank’s health. You may want to repeat this every year. Good, well looked-after service banks can last over 7 years, with little degradation.
    7. Note that after you terminate the C/10 or C/20 discharge, you should see the battery voltage recover somewhat back up to closer to 2V per cell. You should now fully recharge the battery bank to finish the process.
  • NB with both test method, if you conclude that you do need to change any of your service bank batteries it is strongly recommended that all batteries are replaced, like for like (or as similar as possible) at the same time.

[(*) For much more in-depth information and training on batteries and chargers, see module 4 of our course: Electrical Control Systems for superyacht engineers.]

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