Renewing Batteries

[ogi]

The time has come when I wish to renew all the domestic batteries on my 11 metre twin screw motor boat. Is there a formula for working out the power required? I have 2 electric flush toilets, a pumped water system, fridge, cabin lights and various instruments.
The batteries are controlled by a stirling battery management system and when berthed in the marina a 240 volt supply is maintained. When I have decided which batteries are required does anyone know of a good, cheap supplier.

 

[sarabande]

no universal formula, but just lots of boring sums. Unfortunately.

You need to know how many amp hours you use in a typical day. So if you have 200 watts of lighting on for three hours daily, that's 0.6 Ahr.

Do this for fridge, instruments, windlass, fans, pumps, radar, radios, chargers.....

A lot of this will be guess work, but none the worse for that if you err on the 'up' side of your estimates.

This gives you a total daily load - which may vary whether you are under way, or moored up, summer or winter. The figure may be in the region of 80 to 120 AmpHours per day - for which you need to provide (at another guess for safety !) about 3 times the battery capacity, because batteries very roughly only have usable capacity of that figure. So you might need a battery (or battery bank) of 240 to 360 AHr capacity.

All that is very quick and dirty, before you start going into the type of battery suitable for use on board.

It sounds as if you have sufficient charging capacity anyway for a bank of that max size.

It's all a bit tedious, but if you are very careful listing all the equipment and its power requirements, you will be in a far far better management position for the future.

FWIW, in a similar position about 6 years ago, I bought ordinary lorry batteries (110s because that was the size that best fitted the battery boxes !) from the high street.

 

[PaulGooch]

The time has come when I wish to renew all the domestic batteries on my 11 metre twin screw motor boat. Is there a formula for working out the power required? I have 2 electric flush toilets, a pumped water system, fridge, cabin lights and various instruments.
The batteries are controlled by a stirling battery management system and when berthed in the marina a 240 volt supply is maintained. When I have decided which batteries are required does anyone know of a good, cheap supplier.

Too many sums to do to work it out on paper. Easier to ask "what do you have now and do they cope" ?

Nothing you've mentioned is going to use a dramatic amount of power, but if you were away from shore power for days on end, you'd obviously need more battery power than if you only went out for a day or two at a time.

At the current time, i'd say the most economical system would be a bank of 110ah leisure batteries. With respect to Sarabandes lorry battery suggestion, things have changed in the past six years and leisure batteries are much cheaper.

 

[DAKA]

The time has come when I wish to renew all the domestic batteries on my 11 metre twin screw motor boat. Is there a formula for working out the power required? I have 2 electric flush toilets, a pumped water system, fridge, cabin lights and various instruments.
The batteries are controlled by a stirling battery management system and when berthed in the marina a 240 volt supply is maintained. When I have decided which batteries are required does anyone know of a good, cheap supplier.

Get a tape and measure the size of the battery box including the height.
Draw a picture of a battery and note where the terminals are, red to top right or top left, round/square.

The most important issue for you is getting batteries the right size to fit in the hole and to save yourself hours of hassle of rewiring/re routing the system .

If the size allows for an upgrade of 135 ahr then take the opportunity to upgrade but expect to pay 30% more for the extra 20 ahr.
The only calculation is how wide are two batteries side by side.