Log in
Article search:

Q & A

battery bank

I would like to have a deep cycle battery bank 3 x 135Ah (405Ah) and one starter battery 110Ah. I found on internet that suggested ratio alternator to battery bank is 20-25%. I would like to ask you about the confirmation please.
The second issue is the localization of battery bank. My boat is traditional narrowboat so the olnly space I can think about is the current place of the batteries under the deck. Do you think it is safe as it is confined space? Currently one leisure heavy duty and one starter batteries are there but with more batteries there would be more hydrogen from charging...
Kind Regards

Asked by: Joanna and Jarek   | 4.25pm, Monday 7 February

WW says:

Firstly, you are right that the alternator should be about 20% of the battery bank size. However, this is only one side- without external control, even a very large alternator will cycle the batteries between 50% discharged (flat) and around 85% (apparent full). As the resistance of the battery rises as it charges, and the voltage increases, the current from the alternator will be limited, so vastly prolonging the time taken to charge- hence why an ammeter often reads high, but soon drops to 5 amp or so charge.
Using an external controller will increase the charge rate, and the final voltage, so getting more out of the batteries, as well as decreasing the charging time. The current input to the battery will be maintained, as the voltage increases. Some units follow the 4-stage advanced charging that you would find on a high quality 230V battery charger.
So, a well-geared 55amp alternator with an external controller (the Adverc/Sterling/Kestral type) would be better than a 90amp alternator with no external control. I would be tempted to fit the smaller alternator, and go for an external controller. it would also reduce the load on the engine, to a proportion, while still ensuring good charging (assuming you would need a maximum charge input in a day, of around 40% of the battery capacity, or, say 100amps).
For the second question, this is more a BSS question, so Rupert may add to this. if the engine room is well ventilated, then it doesn't count as part of the accomodation. Hopefully, the amount of hydrogen produced would be kept to a minimum, though charge controllers can incrase the gassing (and water consumption) as a byproduct of faster, deeper charger.
Hydrogen is, of course, lighter than air. As long as there is a route out at high level for the hydrogen, this shoudl be fine- if in doubt, provide some ducting to assist this, or a second vent just forward of the counter (or one on either side) to increase the ventilation. If the area is not enlosed, it is not confined- and if it shares the sapce with the engine, if there is sufficient ventilation for the engine (which would be running anyway while the engine is charging) then there would be enough for the batteries.

Mark Langley  | 2.19PM, Tuesday 8 February

To see if the Sterling controller is working, using a digital multimeter, directly at the batteries, measure the voltage over a period of time- it should boost to around 14.8V at peak charging, then settle down to a lower value, around 14.4 or 13.8V.

Mark Langley  | 4.23PM, Tuesday 8 February

The location of the batteries is important; they must be easily accessed for maintenance, well ventilated (as Mark says), and be secure. They must also have insulated terminals or an insulating cover that does not trap any released hydrogen.
It is also worth considering the location with regard to the expected use of the boat. Batteries stuck on the swim under the deck of a cruiser NB are fine for holidays, but do not expect them to perform well in winter conditions. If the boat is to be used all year round, the batteries need to be kept warm. Consider putting them in the cabin space.

Rupert Smedley  | 10.47PM, Wednesday 16 February

Readers say:

Thank you very much.
I found there already small Sterling controller just was not sure if it is working. I am waiting for BSC inspection update and hopefully in 2 weeks there will be new home.

Joanna and Jarek   | 3.40PM, Tuesday 8 February

You must log in to post an answer.