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Old 18-12-2017, 04:15   #1
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Alternator Modification...??

On my 42 foot Sailboat the Volvo D55 Diesel engine has a standard 115A Mitsubishi Alternator. I've three 100A Trojan AGM House Batteries. I've loads of Solar PV..(700watts) BUT on dull days my batteries are not getting FULLY (14.2v) charged so I run the engine to do a 'top up' (maybe the voltage would be 12.9 to 13.5v). I'm pretty sure that it's very important that the batteries do get FULLY charged on a regular basis so that's why I'll give the extra charge. I will point out here that if I start the engine in the morning after the fridge/freezer etc have been running all night and voltage is then typically about 12.4v the Alternator throws out 40A.

Trouble is, because the alternator can 'see' that the batteries are near full it only trickle charges.... putting out around 8 amps or so. This means that I have to run the engine at 1500 RPM for over an hour just to get from say 13v to 14.2v. I know I could buy either an aftermarket Regulator or a Balmar etc BUT, I'm thinking what if I intercept the Field circuit and fit something like a Variable Rheostat to 'trick' the Alternator into thinking the batteries have less charge in them than they actually do? So set the Rheostat to achieve say 30A charge (I've a panel meter).

Once the voltage reached 14.2v (ie fully charged) I could shut the engine down. Any Technical folks out there who know if this is do-able? Maybe It doesn't work like that and I'd end up frying....everything!? I've a sneaky suspicion you'll say I'm mad! Thanks' in advance.
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Old 18-12-2017, 04:37   #2
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Re: Alternator Modification...??

Quote:
Originally Posted by mackerel man View Post

Once the voltage reached 14.2v (ie fully charged) I could shut the engine down. Any Technical folks out there who know if this is do-able? Maybe It doesn't work like that and I'd end up frying....everything!? I've a sneaky suspicion you'll say I'm mad! Thanks' in advance.
Merely reaching 14.2V is not really anywhere near fully charged. 14.2V would also be a rather low absorption voltage leading to unhappy & undercharged batteries over time. A minimum absorption voltage for Trojan AGM's, in deep cycling use (PSOC use), would be 14.4V and these batteries can be charged as high as 14.6V to 14.7V which is a lot more optimal. For PSOC use the highest allowable absorption voltage (temp compensated) is going to be the most optimal to keep sulfation at bay.

Once the batteries exit bulk and attain the voltage limit (absorption) they still have another 3-5 hours to go before they actually become fully charged. At 14.2V you are only charging to the bulk/adsorption point in the SOC curve.

With engine & solar is is most optimal to run the alt in the AM or when ever your batteries are most depleted. If your solar controllers absorption voltage is set higher than the alt, by even a hundredth of a volt, simply install a switch to turn PV off when bulk charging or the two regulation circuits can cause ping ponging which can slow down bulk for the alt.

Also keep in mind that many newer alts have built in thermal protection circuits that can fold back regulation voltage as they heat up, in order to self protect.
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Old 18-12-2017, 04:48   #3
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Re: Alternator Modification...??

Quote:
Originally Posted by mackerel man View Post
On my 42 foot Sailboat the Volvo D55 Diesel engine has a standard 115A Mitsubishi Alternator. I've three 100A Trojan AGM House Batteries. I've loads of Solar PV..(700watts) BUT on dull days my batteries are not getting FULLY (14.2v) charged so I run the engine to do a 'top up' (maybe the voltage would be 12.9 to 13.5v). I'm pretty sure that it's very important that the batteries do get FULLY charged on a regular basis so that's why I'll give the extra charge. I will point out here that if I start the engine in the morning after the fridge/freezer etc have been running all night and voltage is then typically about 12.4v the Alternator throws out 40A.

Trouble is, because the alternator can 'see' that the batteries are near full it only trickle charges.... putting out around 8 amps or so. This means that I have to run the engine at 1500 RPM for over an hour just to get from say 13v to 14.2v. I know I could buy either an aftermarket Regulator or a Balmar etc BUT, I'm thinking what if I intercept the Field circuit and fit something like a Variable Rheostat to 'trick' the Alternator into thinking the batteries have less charge in them than they actually do? So set the Rheostat to achieve say 30A charge (I've a panel meter).

Once the voltage reached 14.2v (ie fully charged) I could shut the engine down. Any Technical folks out there who know if this is do-able? Maybe It doesn't work like that and I'd end up frying....everything!? I've a sneaky suspicion you'll say I'm mad! Thanks' in advance.
I used a welding rheostat for years to set the charge rate on my alternator with no problems. You do need a high wattage rheostat that will carry a lot of current.
I did have a switch to be able to set the alternator back to the regulator if I was going to be motoring for an extended period.
One thing is that the amps I set it for were 10% of the rated capacity of the battery bank.
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Old 18-12-2017, 05:03   #4
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Re: Alternator Modification...??

I stated 14.2 as that seems to be as far as it goes.... even when motoring long distance but then there's stuff being drawn off right? You didn't say about the Rheostat? what's your opinion on that please. I bought my last boat from Rockland, Maine, lovely area :-)
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Old 18-12-2017, 05:51   #5
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Re: Alternator Modification...??

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Originally Posted by mackerel man View Post
I stated 14.2 as that seems to be as far as it goes.... even when motoring long distance but then there's stuff being drawn off right? You didn't say about the Rheostat? what's your opinion on that please. I bought my last boat from Rockland, Maine, lovely area :-)
I guess I should have qualified my response, I was working with lead acid wet cell batteries, which I am familiar with. If you are using gel batteries, they might have a different charge rate. 14.2 seems like a good number with lead acid batteries.
I don't have any current knowledge of rheostats that are available, but that is what google is for. I do know that you need a high wattage one to be able to handle the current. I used a welding rheostat and it served me well for over 5 years. It was large, but space was not a concern, and they can be mounted remotely.
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Old 18-12-2017, 06:54   #6
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Re: Alternator Modification...??

Quote:
Originally Posted by mackerel man View Post
On my 42 foot Sailboat the Volvo D55 Diesel engine has a standard 115A Mitsubishi Alternator. I've three 100A Trojan AGM House Batteries. I've loads of Solar PV..(700watts) BUT on dull days my batteries are not getting FULLY (14.2v) charged so I run the engine to do a 'top up' (maybe the voltage would be 12.9 to 13.5v). I'm pretty sure that it's very important that the batteries do get FULLY charged on a regular basis so that's why I'll give the extra charge. I will point out here that if I start the engine in the morning after the fridge/freezer etc have been running all night and voltage is then typically about 12.4v the Alternator throws out 40A.

Trouble is, because the alternator can 'see' that the batteries are near full it only trickle charges.... putting out around 8 amps or so. This means that I have to run the engine at 1500 RPM for over an hour just to get from say 13v to 14.2v. I know I could buy either an aftermarket Regulator or a Balmar etc BUT, I'm thinking what if I intercept the Field circuit and fit something like a Variable Rheostat to 'trick' the Alternator into thinking the batteries have less charge in them than they actually do? So set the Rheostat to achieve say 30A charge (I've a panel meter).

Once the voltage reached 14.2v (ie fully charged) I could shut the engine down. Any Technical folks out there who know if this is do-able? Maybe It doesn't work like that and I'd end up frying....everything!? I've a sneaky suspicion you'll say I'm mad! Thanks' in advance.
A possible fix if your alternator has a external sense cable (Valeo/SEV and the clones usually do) is to connect that via a schottky diode to the positive pole on your battery bank. And add a switch to bypass it (add in parallel to the diode)
The voltage drop over the diode (0.3V) will make the sense circuit in the regulator regulate the charge voltage 0.3V higher than the actual bank voltage.
Careful watch should be kept to make sure not to overcharge and damage the bank.
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Old 19-12-2017, 09:09   #7
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Re: Alternator Modification...??

Many years ago there was a product called AutoMAC that was sold by Spa Creek that did something very similar to what you are suggesting.

Here is a poor quality copy of the AutoMAC Manual:http://svpegasus.org/wp-content/uplo...5-15-22-03.pdf it may help if you go this route.

If I read your first post correctly, simple putting a Pot or variable resistor in your field wire will not achieve your desired result. In fact it will only reduce the field current and reduce the alternator's output.

What you need is a parallel circuit to the voltage regulator that includes a constant 12v supply + a variable resistor going to the field wire.

Be careful, AutoMAC had a nasty reputation of working "too well", and that was with flooded batteries that can take a lot more abuse than your AGM's.
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Old 19-12-2017, 09:17   #8
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Re: Alternator Modification...??

What I wrote above in my last post is only accurate if you have a high side or P type voltage regulator.

If you have a low side or N type regulator the parallel circuit does not need a 12v supply, just a variable resistor between the field wire and ground. The alternator is already supplying the 12v source.

You need to find out what type of internal regulator your Mitsubishi alternator has before you proceed.
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Old 19-12-2017, 14:03   #9
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Re: Alternator Modification...??

if your 115a alt is only putting out 40a. either the batteries are not that low, or you have bad wiring setup.

you don't run the alt to "top" them up. you would run it first thing in the morning. alow the alt to put out max amps into the drained batteries. then let the solar top up all day long. if you are trying to top up with alt at the of the day. with batteries near full. it isn't going to do much. and yes no alt will do much at idle. it'll always need to be reved up.

of you want to use your alt as a main source of charging. modify it for external feild, and get an external reg.

make sure you are directly charging the house bank. no diode isolators.
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Old 19-12-2017, 22:37   #10
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Re: Alternator Modification...??

Quote:
Originally Posted by mackerel man View Post
On my 42 foot Sailboat the Volvo D55 Diesel engine has a standard 115A Mitsubishi Alternator. I've three 100A Trojan AGM House Batteries. I've loads of Solar PV..(700watts) BUT on dull days my batteries are not getting FULLY (14.2v) charged so I run the engine to do a 'top up' (maybe the voltage would be 12.9 to 13.5v). I'm pretty sure that it's very important that the batteries do get FULLY charged on a regular basis so that's why I'll give the extra charge. I will point out here that if I start the engine in the morning after the fridge/freezer etc have been running all night and voltage is then typically about 12.4v the Alternator throws out 40A.

Trouble is, because the alternator can 'see' that the batteries are near full it only trickle charges.... putting out around 8 amps or so. This means that I have to run the engine at 1500 RPM for over an hour just to get from say 13v to 14.2v. I know I could buy either an aftermarket Regulator or a Balmar etc BUT, I'm thinking what if I intercept the Field circuit and fit something like a Variable Rheostat to 'trick' the Alternator into thinking the batteries have less charge in them than they actually do? So set the Rheostat to achieve say 30A charge (I've a panel meter).

Once the voltage reached 14.2v (ie fully charged) I could shut the engine down. Any Technical folks out there who know if this is do-able? Maybe It doesn't work like that and I'd end up frying....everything!? I've a sneaky suspicion you'll say I'm mad! Thanks' in advance.
In one of his sailboat maintenance books, Don Casey recommends adding a switch with a diode across the poles to the regulator sensing circuit of the alternator. With the switch off, the circuit reads the battery voltage minus the 0.6 volt drop across the diode, increasing the alternators output. Flip the switch to the on position and the regulator circuit bypasses the diode and reads the true battery voltage (for when running the engine longer)
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