GOT THE POWER?
You cruise several hours a day, you’ve got a decent alternator, a battery monitor and an advanced charging manager – but still you keep killing batteries. That’s when a power audit and charging calculation could help to get to the bottom of things…
How do you know if your battery bank and charging system are adequate? Do a power audit and charging calculation
This article began as a question from boater Val to our ‘Ask the experts’ column. But following Tony Brooks’ initial response and request for further information from Val, it developed into something that we felt deserved an article of its own: a fullyworked ‘real life’ example of how carrying out a power audit can highlight shortcomings in an electrical system which at first sight might seem to provide adequate power and charging capacity…
Val bought her boat back in 2012, knowing that the domestic batteries needed changing. But then six months after buying new ones (7 x 110Ah), performance started to decline again. An engineer confirmed that the alternator and cabling were fine, but the batteries weren’t holding a charge – so they were changed again.
She then started to change things in the boat: fitted a new 240V fridge freezer, fed by a Victron 3000W inverter, and bought an automatic washing machine. Then the engine was changed for new one which had a 175A alternator, and at the same time, Val fitted a Adverc advanced battery management system and a battery monitor – plus 3 x 150W solar panels. The third set of batteries lasted for 14 months before they too would not hold a charge.
And all this is despite running the engine for approximately five to six hours per day. She ended up on her fifth set of batteries, taking refuge in a marina with shoreline supply because once again they were
failing to hold charge – and was considering increasing the size of the battery bank to eight, and stepping up from 110Ah to 135Ah to see if this would extend their life. At this point she contacted Canal Boat’s Ask the Experts column…
Tony Brooks’ initial response was to ask for detailed figures to enable him to do a power audit and charging calculations, but also to make two immediate comments, adding the advice “On no account buy more batteries until this is checked: you could easily destroy them in weeks.”
Firstly, an advanced battery management system can help hide problems in a charging circuit and will reduce the time taken to get the batteries to maybe 80% of fully charged, but after that it is unlikely to reduce the time to 100% charged. I doubt your charging circuit had any faults.
Secondly, in general your type of battery monitor has a long and disreputable history of encouraging the less electrically aware boaters to destroy their battery banks by over discharging them and never fully charging them. Unless you fully understand how to set the meter up and when and how to resynchronise it I urge you to only use its voltage and current scales. It is also safe to use the Amp hours used scale to calculate the current battery capacity.
On receiving the figures, Tony then went on to do the power audit and charging calculations. But first an explanation of how batteries degrade, and what you can do to slow the process…
If you do not very regularly fully recharge your batteries a chemical change known as sulphation will take place that gradually reduces their capacity. All batteries sulphate in time, but that time can be a few weeks if they are never properly charged. Also if you regularly deeply discharge batteries, you will quickly use up their cyclic life resulting in internal short circuits.
Both sulphation and short circuits result in the batteries going flat far faster then they did when new, so the question is how do you minimise these effects. Dealing with sulphating is easy: fully recharge very regularly, at not more than weekly intervals. Loss of cyclic life can be reduced by never fully discharging the batteries: try to never drop below about 50% of fully charged.
The easiest way for an ordinary boater to differentiate between sulphation or running out of cyclic life faults is to look into each cell when on charge (use eye protection for acid splashes). If any are drier than the rest or if any are bubbling a lot more than the rest then these cells are likely to be shorting and they are also likely to be hotter. If you measure and record the specific gravity/relative density of the electrolyte in each cell and then compare it to the state of charge shown on a voltmeter they should both indicate a similar state of charge. If the voltage indicates a higher charge than the specific gravity then the cell is sulphated. If the ends of a battery are bulging out it also indicates a sulphated battery.
How do you know how fully charged the batteries are? The way to make sure they are fully charged is to charge until the charging current at 14.4 volts or more has dropped to about 1% to 2% of battery capacity or has stopped dropping for about an hour.
You can use the batteries’ rested voltage to infer the state of charge. Rested voltage means being off charge for at least an hour, or being off charge and a reasonable load has been drawn for a short while. You also need to ensure the solar array is covered or its dark otherwise the solar voltage will distort the figures. Typical values are shown here:
Now on to the power audit. Tony took the figures supplied by Val which gave the amps drawn (from the labels on the equipment, if necessary dividing watts by volts to give amps) and hours used per day for each electrical appliance on the boat. These were multiplied these together to give the amp hours for each appliance, and then added then up to give the total amp hours used per day.
Tony admitted to being sceptical about some of the figures: “I also suspect some of the times are a bit high, but let’s take those
figures as correct for now.” He continued…
That total of almost 282 Ah may sound well within the capacity of 7 x 110 Ah batteries – but remember the comments above about avoiding sulphation and short circuits. You should try not to let the batteries fall below 50% of fully charged to optimise the cyclic life so the minimum bank size for 282 Amp hours of load per day is 564 Ah. However they will still sulphate and it is often claimed that to allow for that it is better to have a daily load of only 25% of battery capacity - so that gives a bank size required as 1128 Ah.
But that’s only the first part of the calculation. You need to make sure that not only do your batteries have enough capacity to last the day, but your charging setup is capable of putting them back to fully charged. And the charging calculation isn’t quite as straightforward as it might seem either…
You cannot get all the current you put into a battery converted to stored electrical energy. You will find the efficiency of conversion (measured in terms of how much extra energy you need over and above the battery capacity) is quoted as anything between 10% and about 40% depending upon temperature, age of the battery, battery design and what the person quoting the figures is trying to sell you. I usually take 30% but as I suspect the daily consumption figures are a bit high I will compensate by using 20% in this case. So the actual number of amp hours from the alternator that it would take to replace your daily usage is 120% of 282 Ah, which makes it 338 Ah.
Alternators cannot produce their maximum output for very long. As the batteries start to charge, the current drops so they can only average about 50% of their output over three to four hours. Any time longer than that simply reduces the average and makes it difficult to calculate.
You alternator is rated at 175 amps so over three to four hours (no more) that averages about 87 amps.
338 Ah taken out and charging at an average of 87 amps = 3.9 hours. But the charging for five or six hours each day looks as if it will get the bank to around 80% charged but not 100% because to fully charge a bank of any significant size is likely to take 12 hours plus (remember go by tail current, not your meter).
So Tony’s conclusions from the calculations were:
Firstly, you have been allowing your batteries to sulphate by under charging although you have done well compared with many with the length of time you charge for.
Secondly, your battery bank is seriously undersized for your electrical load so it is almost certain that you have been over discharging them and have used up the cyclic life.
If, as you suggest, you go up to 8 x 135 Ah = 1080 Ah, that would be more than adequate for having the bank twice as large as the load – but still not large enough to meet the common rule of thumb of four times the load. Remember that as time goes on the actual capacity of the bank will drop so with twice the load you will start to discharge them to below 50% within a few months.
Do remember, though, that is based on theory that has a lot of variables in it, so take my workings as indicative to give you an idea of what is probably wrong.
1128 Ah of batteries is expensive and even more so as you need a decent cyclic life. If you can get to all but fully recharge every day or to about 80% (not on the meter) and then 100% once a week then I would suggest that you cost some deep cycle batteries, Trojan 105s are well spoken of but they are 6 volts so need pairing in series to give 12V. If you cannot ensure proper charging then stay with relatively cheap wet open cell batteries, and accept that with your loads and charging you will need to change them frequently.
I also think that you need to take a very hard look at reducing the load:
• Why anyone is using halogen bulbs today is beyond me, especially when they have battery problems.
• I have no idea why a boat needs a kitchen pump but not a shower pump. The sink normally exits through the hull by gravity.
• Unless you have a very cheap fridge your fridge consumption looks very high to me. I would expect it to be 10 to 20 Ah lower.
• I can not see how a Webasto motor and fuel pump can use more electricity than a water pump. From some data I found online: - Pump motor 14 watts, fan motor & fuel pump at full load 28 watts = 42 watts so 42 watts at 12 V = 3.5 Amps, that is 27 Ah and part of the time it will be on part load so reducing that figure. In truth it is probably only drawing about half of what your power audit suggests. • I also doubt the toilet runs for 3 hours a day. Finally, to try to pull this all together: You need a new bank so put it on, but monitor it very closely. 5 X 135 quality batteries will do for now.
Recharge each day until the charging current has fallen to about 1% to 2% of the bank capacity (so for 5 x 135Ah that would be 6 to 12 amps) at 14.4 volts or more. Zero your amp hour counter. Read the Ah counter before starting the next day: that will give you your actual consumption without the engine providing any electricity.
From that you can work out the bank capacity you need for optimum life and act accordingly but I emphasise that you need to fully recharge as often as you can and at no more than weekly intervals.