Combining Thruster Battery with House

[DBBRanger]

Many tugnutters have noted that they have combined their 2 house batteries and their thruster battery in order to get more house capacity. I have done this by adding a battery combiner switch so that I can easily "uncombine" them if desired. So I put in a battery switch with inputs of thruster battery (from thruster bus bar) and the other input from the house battery (from house buss bar). The output goes to the thrusters. The switch is usually in the "Both" setting, thereby combining house and thruster batteries that supply the thrusters but more importantly supply the house needs as well. If desired, I could also supply the thrusters with either the thruster battery OR the house batteries. See the schematic below.

I came back from a 3 month trip and added water to the batteries. The two house batteries took a substantial amount of water, the thruster battery took almost nothing and the starter battery needed no water. That seems to tell me that the thruster battery was not used much and therefore did not get charged much. When I use my multimeter to check current going through each battery, the total current is shared by the three batteries as it should be as well as the charging current is the same for each of the three. Also, my Victron battery monitor (using the shunt shown in the diagram) could be read in a way that also says I'm not using the thruster battery since it can show 75% SOC but the voltage is 12.3 volts when resting. It seems the voltage should be higher to match a 75% capacity when using three batteries.

So the question is why am I losing water in the house batteries but not the thruster? Any ideas?

 

[HRowland]

How old were the batteries when you started combining the house and thruster bank? I suspect your house batteries are approaching end of useful life while the thruster battery has not had many discharge/charge cycles. Likewise the engine battery never is discharged much in normal use. When you replace the house and thruster batteries you should see similar water usage.

I assume you reprogrammed your Victron with the larger capacity of your combined bank. Also note that the Victron is not aware of battery aging and capacity loss and will start to mislead you on charge status. Older batteries act like a smaller AH capacity battery. I also have a Victron with a permanently combined house/thruster banks. I added a new bus bar for +12V so each battery has equal length cables and all loads/charge sources are at the exact same potential.

Howard

 

[SGIDAVE]

Hello Doug,

I suspect the water consumption differences of your 3 batteries relate to differing battery ages and/or the way they are connected into one bank.

I can't tell if your 3 battery bank (consisting of batteries #2, 3, and 4 in your diagram) are 'balanced.'

By 'balanced' I mean that the POS wire comes from battery #2 and the NEG wire from #4 (which would terminate at the shunt). Basically, you are trying to make the "juice" flow from the POS at one end of the bank to the NEG at the other end of the bank. What you DO NOT want is for the POS and NEG cables to come off the same battery in a multi-battery bank.

Here is a 2 battery bank set up the way I'm describing:

Imagine this setup with a third battery.

Other than the factory wiring the boat with a house bank of (2) 12V batteries and two additional 'banks' of single 12V batteries...why not PERMANENTLY create a bank of (3) batteries for house and thruster loads? It would significantly simplify your wiring by eliminating the switching. I can't imagine a scenario when having house and thruster loads on one, larger battery bank would be a disadvantage. On the other hand, having them split is definitely a disadvantage...from a safety perspective others have described having the thruster battery voltage drop below 10V with significant thruster usage...which causes the Blue Seas ACR to OPEN...which separates it from the charging current from the alternator (the ACR is supposed to do this) and you end up with insufficient battery 'juice' to operate your thrusters when you are in a difficult situation. Having a (3) battery bank from which to power your thrusters (and house loads) would ALWAYS be advantageous. Ah well, just a few musings on battery setups....

fair winds,

dave

 

[DBBRanger]

Thanks for the replies. The batteries are 48 months old. I combined them when they were 28 months old. In addition, the thruster battery is a combination starting and deep cycle type but the same capacity (90amphrs) and brand as the house batteries (the manufacturer said it would be OK to combine them).

I did reprogram the Victron for the 3 battery condition.

I don't have them 'balanced' per the diagram. Each has a dedicated negative to the shunt and the plus side is as drawn.

So it seems like you guys are suggesting that even though there is a difference in water usage, there still may have been a relatively equal (or at least similar due to the unbalanced design) amount of power drawn from each battery and therefore I may still have been able to benefit from the additional capacity? I guess I don't understand the connection between water usage and age - the older the battery, the more water will be used?

 

[NorthernFocus]

Not sure which engine you have or how you are wired. But one potential issue if your boat is wired like mine is that the ECM may get its power from the no.2 battery. If you draw them down too low you may not be able to start your engine. Not sure what that threshold is but it is something to be aware of and to check out. I've got the Mercruiser/Cummins engine and the ECM is particularly sensitive to battery voltage during start.

...I guess I don't understand the connection between water usage and age - the older the battery, the more water will be used?

What really consumes water in lead/acid batteries isn't usage but rather over charging. Once the battery reaches a state of full charge then water begins to cook off. However, old/well used/abused batteries effectively have lower capacity than newer/better condition batteries. So the old/lower capacity battery can actually reach a state of full charge quicker. If the batteries don't have the same history they charge/discharge at different rates. So if your no.2 battery is in better shape than the house bank, yes the house bank will use more water given the same amount of usage and same time on charge with the same voltage being applied(which is how the engine alternator charges i.e.with a simple voltage regulator).

The threshold at which traditional lead/acid batteries begin to suffer from sulfate damage is nominally 12.3V. Each time they are pulled below that level they suffer some level of irreparable damage and lose some capacity(i.e.amp-hours of storage).

But I'm no expert on the topic. This is my understanding and seems to jive with my historical experience.

 

[DBBRanger]

Not sure which engine you have or how you are wired. But one potential issue if your boat is wired like mine is that the ECM may get its power from the no.2 battery.

No ECM here, I've got the Yanmar 110HP

What really consumes water in lead/acid batteries isn't usage but rather over charging. Once the battery reaches a state of full charge then water begins to cook off. However, old/well used/abused batteries effectively have lower capacity than newer/better condition batteries. So the old/lower capacity battery can actually reach a state of full charge quicker. If the batteries don't have the same history they charge/discharge at different rates.

So, since I have a 3 bank battery charger, can I just uncombine the house and thruster batteries when I charge them so that each bank gets just what it needs? I can never remember if the chargers (ProNauticP 1240) are that smart. When the alternator is charging and uncombined, then the ACR's will first charge the thruster battery then the house. So maybe I might use more water in the thruster battery?

 

[NorthernFocus]

...So, since I have a 3 bank battery charger, can I just uncombine the house and thruster batteries when I charge them so that each bank gets just what it needs? I can never remember if the chargers (ProNauticP 1240) are that smart...

If you have the older version of the ProNautic(i.e. the one originally supplied with the boat) then I don't believe they are that smart. For sure not smart enough to prevent cooking off excessive water. At least that was my experience. Due to that issue I removed mine and installed a simple trickle charger just to simplify my system. The trickle charger still cooks the batts unless I force the ACRs open. They claim that the newer versions of the multibank chargers are that smart but I don't care because I actively manage my power anyway. It gives me a greater sense of self worth and false sense of security doing it myself.

... When the alternator is charging and uncombined, then the ACR's will first charge the thruster battery then the house. So maybe I might use more water in the thruster battery?

I don't think so. At least not based on how the ACRs function on my boat. They open long before batteries are topped up. And once they open the amps follow the path of least resistance(i.e. to the lowest battery charge).

But how/why batteries use more/less water is way over my head. Last year I installed two brand new batteries ganged together as a single bank from day one. When I add water to them there are noticeable differences between individual cells. Presumably do to variability in the plate manufacturing process but that's a guess on my part. When I was younger I would have spent hours investigating the why's and wherefore's. Now as long as things are working I just chalk it up to another one of the mysteries of life and move on to the coffee pot and sofa :?

 

[DBBRanger]

They open long before batteries are topped up. And once they open the amps follow the path of least resistance(i.e. to the lowest battery charge).

I'm confused with your nomenclature. The ACR CLOSES when charging (V>13.0 or 13.6) connecting the two batteries and OPENS when discharging (<12.35 or 12.75) disconnecting the two batteries. Thus, I don't understand what you are saying.....

Thanks for the reply

 

[NorthernFocus]

That's correct, the ACR switch closes. No idea what guided my fingers to type OPENS.

 

[doke01]

Batteries at 48 months are near end of life in my experience. I end up replacing them every 3-4 years. I believe most batteries last that long, unless you want to spend an excessive amount of money for a high end battery. I never saw the cost advantage of doing that though. I've never have had to add water unless the battery started to go bad.

 

[Red Raven]

FYI. When the thrusters are activated there can be a significant voltage drop due to the high current draw. This voltage drop can cause the Garmin GPS to lock up. I don't know if any of you who have combined it with the house have experienced this but that is why the thruster battery is wired independent according to RT.

 

[HRowland]

I combined the banks at the beginning of 2013 and never had any issue. I also improved the battery current balancing by using 600A bus bars, one for +12V and one for ground. All house/thruster/charge sources connect directly to the bus bar, each battery has dedicated 1G jumpers to the bus bars. Another item to remember is that the engine is typically running when using thrusters, I have seen the alternator on the 4BY2-180 deliver well over 100A at idle (I have a Victron monitor installed, one of its features is current measurement) This significantly helps with the thruster load. On my R27 the engine alternator is directly connected to the house bank, the Volvo powered tugs may be different, if connected to the engine battery it is possible for the ACR to open due to voltage drop and the alternator would not be able to help with the thrusters.
Even when using both thrusters simultaneously I have never had any issues.
While it is possible to drop the voltage so low that the Garmin or other devices may drop out, the batteries would have to be quite depleted for this to happen. NMEA-2000 devices are designed to operate down to 9V.

In my use case, and I suspect most others, the batteries are typically fully or nearly fully charged when using the thrusters. The only time my batteries are significantly drained is after anchoring or being on a mooring. When leaving thruster use is not typically needed, and by the time I am back at a marina for docking the batteries are mostly charged. If I am leaving a dock the batteries are typically fully charged, this is the other time I would use thrusters.

But, the factory warning is something to consider. I was told the same thing.

Howard

 

[Red Raven]

Thanks Howard. Good to know you haven't seen any issues. I have been considering combining them but don't want to cause problems. I will check the alternator connection on our boat. Thanks for the tip.

 

[knotflying]

Thanks Howard. Good to know you haven't seen any issues. I have been considering combining them but don't want to cause problems. I will check the alternator connection on our boat. Thanks for the tip.

You can always play it safe and install an on/of switch between the setup and always isolate the thruster battery from the house. As I sated on another thread and per Howard, I never had an issue with the Garmin locking up or having such a drain that other equipment failed. Usually when docking or leaving all batteries are well topped off. As a matter of fact I would say the thrusters operate much better. In the past with only one battery and a lot of thruster was needed you could tell the battery was feeling the pain.

 

[Red Raven]

Thanks Mike,

I'm going to see how well my current setup works with two solar panels now that I have got that working properly. My main reason to post was to document the reason the thrusters are wired independent so that those considering this change have full information and take that into consideration.