Adding a second battery - what size?

[Poignard]

At present I only have one battery (105Ah). When that is flat I have to get the starting handle out. The time has come to fit a second battery and a changeover switch.

What size battery should I fit? Another 105Ah or something smaller, to be used only for engine starting?

The boat is a 28 foot sailing boat with a 10hp BUKH engine.

The electrical load consist of navigation lights, cabin lights, echo sounder, log, GPS, VHF radio.

There is no fridge and the Tiller-pilot is only used when the engine is running.

 

[Tranona]

For a start battery CCA is more important than amps. So if your current battery is sufficient for your domestic needs a basic car battery of sound 65 ah is sufficient if you only use it for starting. Alternatively you can pay twice as much for an AGM such as a Red Flash. If however you want it to double up as a regular service battery then another 105 ah leisure battery would be suitable.

 

[FullCircle]

If you are flattening the batteries like that, can I suggest
2x105ah leisure and a 70ah high CCA start battery.

But most importantly perhaps, a charging regime that prevents extended discharge periods.

Small wind genny or a 20w Solar would help a lot.

 

[jac]

Think about how you want to manage the batteries. Traditional way was via a 1 2 both off switch and use either bank for anything. More modern way is to have a dedicated engine start with decent CCA rating, a battery bank for domestic use sized to cover your likely needs and then control charging automatically.

If you do go the automatic way, make sure you have a way of starting the engine if engine start is flat. As you can hand start that may be enough, especially if you can do it from cold.

 

[wooslehunter]

If your existing battery is a conventional one, get a "leisure" battery. If you regularly discharge the existing one, then get a bigger one.

Conventional car batteries are designed to provide a very high current for a short time. i.e. to start the engine. They are not designed to be discharged deeply. Regular deep discharge of a car battery will cause a reduced life. Leisure batteries are designed to provide a much smaller current over a longer time & to be discharged to a level way below car batteries. A leisure battery can be used for engine starting once in a while.

So, using a car battery for the engine & a leisure battery for everything else makes sense.

 

[blackbeard]

At present I only have one battery (105Ah). When that is flat I have to get the starting handle out. The time has come to fit a second battery and a changeover switch.

What size battery should I fit? .......

generally, the answer will be something like:
domestic battery: as much capacity as you can reasonably fit in, probably leisure batteries unless you want to pay for deep discharge ones
engine start battery: something with enough cranking amps to start the engine easily, will probably be a car type.

for the record, my own boat has a couple of 75 Ah leisure batteries as domestic supply (would fit more but not enough room on a small boat!) and a 40 Ah 440 cranking amps car battery for engine (Yanmar 1 GM, single cylinder diesel with 1 kW starter motor), either bank will start the engine. I might have used a bigger battery for engine start - if I could have found somewhere to put it!

if you find you are regularly flattening the existing battery, it's probably well down on capacity. Which is likely if it's been flattened a few times, or indeed often left with less than full charge.

 

[ghostlymoron]

Ideally you need to do an energy audit. That is add up all the drains on the battery with the duration of the load and that will tell you how much battery capacity you need. Remember that the stated battery Ahr figure should be halved to prevent it being discharged below 50%. The next thing to consider is how to replenish the charge - engine, solar or shore power. If you're on a mooring, you'll need to be self sufficient which will limit the amount of battery capacity it's worthwhile installing, if on shore power, you can pretty much install as many batteries as you like. The amount of power you draw is therefore limited by the amount of charge you can produce.
On my Mirage 28 I had one 105 Ahr for domestics and one 85 Ahr for engine starting which I found quite enough to power the lights, instruments, plotter and Eberspacher. Charging was from the alternator (about 1 hr/day) and a 20w solar panel, the charging being controlled by a volt sensing relay prioritised to the start battery so I could always run the engine. I had jump leads to allow starting the engine from the 'service' battery if necessary. I never used them but a crossover switch would have been more convenient.

 

[aquaplane]

My Centaur has the same ammount of demand but I already had 2 batteries and a 1/2/both switch.

Last season I swapped a knackered 110Ahr lorry battery for 2x100Ahr leisure batteries with a smaler footprint on the domestic side.

I also fitted a 40W solar panel which was usefull in the summer but wasn't doing enough when I went over the new year break, I should have removed the domestic batteries and taken them home over the winter.

You could certainly use a dedicated cranking battery but modern lorry batteries tend to have a construction which can cope with both types of demand and the extra domestic capacity I found handy.

 

[Poignard]

Many thanks for the comprehensive advice.