Battery Bank Questions

[C08]

I have two banks of 110AH leaisure batteries charged by a Rutland WG and two solar panels 0f 80W each which enable is to be power independant when at anchor etc. A query.
I have a 60AH starter battery for a bow thruster which I keep charged by connection to one battery bank only when not using via a key switch connection. That is just by a light twin cable with a 10A fuse as I wished to avoid having to run 2x 11M heavy cables from opposite corners of a catamaran. It seems to work well and I keep the BT battery ready to go all times. I am wary of connecting the bow thruster battery to the main bank if there is a large difference in voltage as I am unaware of the current which may exceed the 10A fuse. How can I calculate the current flow from the two 110 batteries to the bow thruster battery. I find difficulty finding the batteries internal resistance?

 

[Tranona]

The draw on a bow thruster battery is low - although the current is high for a short period it does not take much of the capacity. When I bought my boat the bow battery was charged by a solar panel. since then I have replaced that with a B2B from the engine start battery. The bow battery is however nearly twice the capacity of yours and serves both the thruster and the windlass. this only needs 10mm2 cables.

If I am right in assuming that you have 2 batteries that serve both engine and domestic with a 1.2 both switch so you can choose to use one or the other and you connect the bow battery to the one not being used so it takes some of the charge from the wind or solar (but not from the alternator). In that case the current flow would be low. However if that is how it works why bother with going through the big battery and not direct from the solar?

 

[C08]

Thanks for reply - I have two banks of 2x110AH and neither is a dedicated house bank or engine bank and I just use for domestic which bank is the higher voltage and I am carefull not to let the voltages get too low ever. When the bow thruster is connected to a bank it is also being charged by the solar & wind genny. I suppose the connection I have is what you are suggesting.
I am still interested in the current calculation B2B?

 

[jac]

B2B is a Battery to Battery Charger - it will typically not change the voltage ( but I think some can) and is designed to replace your set up by adding some intelligence into the set up. Different models are configurable in different ways so it depends on what you want.

Key point is that it will regulate the charge to the battery

 

[Tranona]

The B2B charger I have is a Sterling and is nominally 30A. Once the start gets to 13.7v the charger diverts charge to the bow battery. It is programmable for any battery profile including lithium, although i have a basic Numax leisure battery so use the default setting. It avoids having a 3 way splitter for the alternator and crucially only neds 10mm2 cabling. I have located it less than a metre from the start battery, which is an Odyssey 28Ah high power (900 CCA) that is charged through a VSR from the alternator and a trickle charger from the house bank. Like most dedicated start batteries it is fully charged within minutes of the engine starting so the bow battery is also quickly topped up. The charger then shuts down.

Fan of dedicated batteries for different purposes and simple ways of charging. I chose this method because of the ease of cabling and enables the use of the BEP switch cluster for the house and start with its built in VSR. Cost wise it is about the same as a 3 way splitter and heavy cables.

 

[C08]

Thanks for the info - it seems what I need is a B2B charger rather than just connecting the bow battery to the domestic bank.

 

[William_H]

No I don't think you need a B2B. Connecting BT battery to a battery being charged should be fine. Except if the BT battery is very low in charge or the thruster is in operation. Then as your concern shows current through the small wire and 10A fuse may be too much. Simple answer is to fit some resistance so current is limited to less than 10 amps. So you may guess at BT battery 11 volts, charged battery at 14v so 3 volts to be dropped at 10 amps. R=E/I so .3 ohms needed. This will dissipate 30 watts. You need 3x 10w rated .1 ohm resistors. Albeit they may get hot. (for short period) You may think BT battery will go lower in voltage so at 10v then 4v dropped need .4 ohm or 4x resistors.
If you are concerned about heat generated or can only find 5w resistors 0.1 Ohm 5 Watt Wire Wound Resistor | Jaycar Electronics fit 2 lots of 6x .1 ohm in series in parallel with another 6 in series. Makes .3 ohm still but with 4 times power capability. Mount the resistors so any over heat will not damage anything nearby. Or alternatively yopu may be able to fabricate a piece of SS plate to make a suitable resistor. Try 20g plate 1.5cm wide by 20 cms long. Need an amp meter and battery to determine actual resistance. Longer and narrower for more resistance.
In the end more resistance means longer recharge time and less boost from domestic battery when thruster is ion operation.. ol'will

 

[PaulRainbow]

Thanks for the info - it seems what I need is a B2B charger rather than just connecting the bow battery to the domestic bank.

You can also fit a VSR, using thicker cables and a bigger fuse.