Charging house bank from alternator?

[FlyWright]

This is a very simple fix. Wire the alternator directly to the start batteries. Then connect the house bank to the start batteries with an automatic charging relay (ACR) sized to take the full alternator output. Problem solved. No fooling with switches. This setup will charge the start battery until its voltage goes above the ACR kick in voltage at which point the house batteries will be connected to the start batteries AND the alternator. When the engine is off the ACR will open and disconnect the start and house batteries. If you have two house banks, put a second ACR between the house banks. That way the alternator will charge all three banks. Simple, fool proof and cheap.

I partially agree but I'd wire the alternator directly to the house bank(s), then use the ACR to share the charge with the start battery. This way you won't overcook your start battery with high charge loads going through it to the house bank.

You can also add a toggle switch to defeat the combiner function if the start battery is full and the house is depleted and receiving a high rate of charge.

 

[john61ct]

I partially agree but I'd wire the alternator directly to the house bank(s), then use the ACR to share the charge with the start battery. This way you won't overcook your start battery with high charge loads going through it to the house bank.

You can also add a toggle switch to defeat the combiner function if the start battery is full and the house is depleted and receiving a high rate of charge.

While the usual advice is alt to house, that can be impractical or expensive, and the reason is most VSRs pass less current than the alt puts out and you don't want thirsty House to miss out. Plus some VSRs may get hot if underspec'd and fail.

If you use e.g. the Blue Sea 500A capacity ACR, that reason goes away, and that switch would just add complexity, no reason for it at all.

Nothing goes "through" the Starter batt, current flows around it to House, no cooking, batts only accept what they can anyway.

 

[kchace]

I partially agree but I'd wire the alternator directly to the house bank(s), then use the ACR to share the charge with the start battery. This way you won't overcook your start battery with high charge loads going through it to the house bank.

You can also add a toggle switch to defeat the combiner function if the start battery is full and the house is depleted and receiving a high rate of charge.

Yeah this is the "new" way which I have read about in several publications and also is espoused by Nigel Caulder. When I revamped my charging system 4 years ago I did it this way and it works extremely well.

Ken

 

[Simi 60]

Toasted our 2nd alternator yesterday

4 month old big frame 70amp alt with a victron Cyrix battery combiner doing house after starts.
It died last year when on a longer trip and I put it down to old age
And yesterday was another all day passage bringing batts from 80% up
Perhaps it's running the alt flat chat for more hours than it likes?

Not happy, but far from the end of the world.

 

[gsholz]

If this is the standard Prestolite/Motorola/Leece-Neville 8MR alternator, on a 3208, then you will want some level of temp protection. The internal regulators on these alternators do not have any sort of thermistor driven voltage reduction circuit and they will literally run until they burn out, if you let them.

They can be converted to external regulation so that current limiting and an alternator temp sensor can be used to protect them.

Leece-Neville 8MR External Regulation Conversion


.

Listen to this advice. My previous boat had a large 12V house bank and was killing the 3208NA alternator regularly trying to charge a depleted house bank. Previous owner replaced alternators several times. I avoided killing the alternators by doing some bulk charging via the generator before starting the engines after a night on the hook.

 

[rslifkin]

Toasted our 2nd alternator yesterday

4 month old big frame 70amp alt with a victron Cyrix battery combiner doing house after starts.
It died last year when on a longer trip and I put it down to old age
And yesterday was another all day passage bringing batts from 80% up
Perhaps it's running the alt flat chat for more hours than it likes?

Not happy, but far from the end of the world.

What component is failing? If it's the rectifier diodes, it's likely getting too hot. Either throttling down the output or improving airflow through it (and supplying cooler air) will help.

All of these stories make me think I should just leave my existing 55 amp Mando alternators alone unless I want to spend big $$$ on better ones and good regulators with temp sensors, etc. These have been working fine for 35 years and even when run hard or flat out for significant periods of time, they don't seem to suffer at all. I'm guessing the output is low enough relative to their size and airflow that they just don't get hot enough to cook. There's a 65A version of the same units available that would be an easy upgrade, but I wonder if they'd be as durable or not...

 

[DDW]

Where did you find a big frame alternator that only puts out 70 amps?

You MUST temperature limit an automotive alternator when charging a house bank, it simply is not made to do this. It is made to replace the < 1 AH used to start the engine, and maintain vehicle loads when running. I'm using a Delco 28si, rated at 160A "continuous" built for bus duty. It might do that in the lab with 150 mph air jet on it, but in the real world of an engine room, limiting the frame to 100 deg C it will quickly warm and can only do about 90 continuous. I've bumped that up (Balmar 614 reg) to 107 deg C and can get about 105 A continuous. Much more than that and life will be severely shortened.

 

[Simi 60]

Where did you find a big frame alternator that only puts out 70 amps? .

Pretty common on 855 Cummins
I got mine from a place called cooldrive
Looks same as this so assume it's big frame, on balmars website they call it "extra large frame"

https://balmar.net/products/extra-large-case-alternators/

 

[Simi 60]

What component is failing? If it's the rectifier diodes, it's likely getting too hot. Either throttling down the output or improving airflow through it (and supplying cooler air) will help.
...

No idea, we are on the water so can't get it checked?

Google indicated it was diodes
Warm days after engine off and screwdriver sticks to pulley (magnetic)

 

[Simi 60]

Listen to this advice. My previous boat had a large 12V house bank and was killing the 3208NA alternator regularly trying to charge a depleted house bank. Previous owner replaced alternators several times. I avoided killing the alternators by doing some bulk charging via the generator before starting the engines after a night on the hook.

External reg sounds like what I need me thinks but running Genset to do bulk almost defeats the purpose of an alt for me.
Large solar which we have will do the batts or may as well run Genset for another hour if needed.

Add: balmars external regulator is $870 on top of a $350 alternator
Buys a fair bit of Genset time

https://www.outbackmarine.com.au/multi-stage-regulator-24-volt-includes-harness

 

[FlyWright]

After I toasted my Balmar alternator, I followed the suggestion of Rod Collins/Compass Marine/CMS and added a dedicated blower with a duct directed to the alternator case. It was a simple solution to overheating after I discovered the other problem...an alternator temp sensor lead that was connected to the wrong point of the alternator. The installer (not me) had used the offset ground lug instead of connecting to the threaded hole in the case which was intended for this purpose on my small case installation.