Adding a third battery

[dvuyxx]

I want to add a second battery to my house side (1). Is it as simple as connecting the two batteries to each other or is there more to consider? I'd like to add an inverter to handle a laptop computer and other modest devices. I am on a mooring ... no shore power. Was also thinking about a solar trickle charger.

[mitiempo]

To create a 2 battery bank (with 12 volt batteries) you would parallel it with the existing battery. It is best not to combine a new battery with an old one though as there is a mismatch and the old will drag the new one down.
It might be a better idea to get a 12 volt adaptor for the computer as it is more efficient.

[sailingdog]

Dvuyxx—

I'd highly recommend you read the primer I wrote about solar power on boats on my blog.

I'd point out that if you're going to be depleting the batteries significantly, as you would with an inverter, you really need more than a trickle charger.

As for adding a second battery to the house side... I would recommend buying TWO NEW batteries. Adding a new one to an old one is likely to drastically shorten the life of the new one.

[Broomfondle]

If you are connecting batteries to make a bank then they really need to be identical. Same type, same age, same useage. In fact you will often end up considering that a single, larger battery might well work out better than any combinations of batteries.

[mitiempo]

Broomfondle
Unless you want a bank of much over 200AH

[sailingdog]

Smaller batteries ganged together often make far more sense from a usability point of view. Lifting a 4D battery is often dangerous and with a lot of risk to injuring one's back...when compared to lifting two smaller batteries. Placing a battery bank with smaller batteries is often easier as well.

[mitiempo]

As well even an 8D won`t give you a very large house bank. Only just over 200AH. While 4 Trojan T105 at 225 AH each will give you a 450 AH 12 volt bank and they are a lot easier to handle as well as find space for.

[Broomfondle]

Aye - good points, well made.

[sailingdog]

Also, if one battery dies out of four smaller 12 VDC batteries, you still have three making up the house bank to use. If one of four 6 VDC golf cart batteries dies, you still have half the bank (2 x 6vdc) to use as a house bank. If you only have one big house bank battery, and the one big battery dies... you're screwed.

Redundancy and resiliency on a boat is a good idea.

[SteveInMD]

When planning what size bank you need remember that you really only have 30% usable capacity in the bank. For example if you have a 100 amp hour bank you have 30 amp hours that's usable. This is because you should never draw the bank down below 50% - so 50 amp hours is your floor. Also, it takes many more hours of charging to charge above 80% than between 50 and 80%, so 80% full is your practical ceiling. Therefore you operate between 50% and 80% most of the time, which leaves you 30% to work with.

If you end up with solar, you may be able to use the 80 - 100% portion sometimes as well, since you will have a lot of charging hours. Of course this assumes the solar system is large enough.