Battery interconnect cable size

[elias]

Hi to all,
I am on adding a 3rd battery on my 2*140ah ( bosch T4) , the battery will be around 85ah. I made some calculations and concluded to a 25mm2 cable for the interconnection . But the present wires are massive that make me think twice . The larger load is my tigress windlass 1200w that is always used with motor on and revs on 1500. So I calculated with a maximum load of 100amps. The interconnection cable is around 40cm.

Any other experience ?

[Salty]

My B36(2002) was fitted with a Lofrans Cayman 1000 watts windlass from new, and that had two Bosch 140 A/H batteries installed when I bought the boat second hand at the beginning of 2010. I don’t know how old those batteries were at the time, but they remained in use until December 2015 when I replaced them with two 220 A/H batteries of lesser quality. The windlass worked just fine both then and now, and likewise with engine running when the windlass was in use. Over winter while the boat is in its cradle on shore, I’m happy enough to use the windlass briefly while ranging the anchor and cable out across the yard for its annual inspection, and with the mains powered battery charger on instead of running the engine.
As for cable sizes, I would stick to using the same size cables that you already have within your battery locker for connecting the batteries together.

[SYJetzt]

I don't know why you want to add a third battery, but it does not make sense to add other batteries than of the same size and age together!
If the windlass seems a little bit weak, you should first check the cabling to the windlass and the solenoid. If there is everything OK, you should think about changing your present battery set.
Adding a new battery of other age and capacity will age the new battery in a short time and has no long term effect !

If you really want to fit another battery, you should under all circumstances  use a cable of the same size as the present ones in your battery locker. You should only use proper cable terminals and the right crimping tools for this job. Otherwise you are risking fire hazard!
When connecting the third battery you should wire all three in parallel (with same cable size and length) and connect the feeding cables to the opposite ends of the bank (for example "Minus" to battery one "minus-terminal" and "plus" to battery three "plus-terminal"). This keeps all three batteries on the same charging state and give you a longer battery life.
If you are not familiar with such a work or don't have the appropriate tools, you should think about going for a professional!

 

[elias]

Thanks for the info , I am quite satisfied with the current setup but I am adding 2 electric toilet motors and more use of a small 375w inverter . The idea was to put the 3rd battery exclusively for the loads of wc , toilet motor and shower drain and charge it trough the solar with extra charger. I aske an electrician and also in this forum that is better to get it simple just adding the 80ah battery to the present bank ( oh the batteries are 5 months old and worked for 2 months ).

[SYJetzt]

For the additional loads like pumps you need no additional battery capacity, because their runtime and the drawn Amp-hours are very small. The only thing which is to think about is the inverter. It will draw 30Amps at full load, which will empty the 80AH battery in a little more than one hour (keep in mind, that you should not draw more than 50 to 60% of your battery capacity, otherwise you will shorten your battery life dramatically).
If you run an inverter from your batteries, i will strongly recommend a battery-monitor system (Nasa, Victron,..) to track your energy-draw from the batteries and to avoid deep discharge of your batteries, which will reduce the battery life.
Your present batteries seem quite new! If you have enough space in your battery-locker, you can extend the capacity by adding another battery of same type, make and capacity without any problem.