Charging batteries from an alternator

cptdondo
cptdondo Solar Expert Posts: 105 ✭✭✭
in Solar Beginners Corner #1
I have an RV question.... I have a pair of Trojan T145s hooked up to some solar panels. They can also charge off the vehicle alternator.

While the solar charger is a "smart" charger, the alternator is not. It holds the voltage at a rock steady 14.4V, which is way too high for the Trojan T145s.

Since I live in the Pacific NW where sunshine is hard to come by 6 months out of the year, I cannot depend on the solar panels to charge the batteries, I need the alternator to charge the batteries when I'm driving.

But.... How do I regulate the voltage? Is there a "smart" regulator I can use at the battery?

I found this:

http://www.powerstream.com/DCC.htm

Which would work, except that 7.5A is not enough to charge the T145s and the cost is astronomical. I'm looking for something simpler...
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Charging batteries from an alternator

    Balmar makes a few of them in the $200 - $400 range. The host here has a couple Solar Charge Controllers that would probably work, you'd have to test them. There is no reason any PWM controller wouldn't work. What you need to do is run the batteries down and put a clamp on Amp meter on the wire from the alternator and see how many Amp's it's putting into the batteries.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: Charging batteries from an alternator
    Blackcherry04 wrote: »
    Balmar makes a few of them in the $200 - $400 range. The host here has a couple Solar Charge Controllers that would probably work, you'd have to test them. There is no reason any PWM controller wouldn't work. What you need to do is run the batteries down and put a clamp on Amp meter on the wire from the alternator and see how many Amp's it's putting into the batteries.
    The only potential (pun intended) problem with using a random PWM controller is that the voltage drop across the CC during the ON pulses may be so high that the 14.4 from the generator would not be able to drive a high enough voltage to the battery terminals. It is possible to get an arbitrarily low voltage drop across a properly designed MOSFET series switch, but I would not count on that from any particular PWM CC!
    If you are concerned primarily with never exceeding the Float voltage of the batteries and are not worried about fast charging or temperature compensation, you could simply switch a few forward-biased diodes into the circuit to drop the voltage. Another approach would be to simply turn off the generator when the battery current declined to level indicating the effective end of Absorb.
    SMA SB 3000, old BP panels.
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #4
    Re: Charging batteries from an alternator

    Thanks. This is in a vehicle, and the batteries charge off the engine alternator, so I'm not at liberty of shutting off the engine when the batteries are charged. I have an automatic charging relay that limits the voltage to between 13.2 and 14.5V, but as long as the voltage is between those limits the alternator is pushing current into the batteries.

    Interesting about the PWM chargers; I have a MorningStar SunSaver Duo hooked up to the batteries, so I could measure the voltage drop across it at 14.4V if I could rig up an appropriate test.

    Any suggestions on the best way to see if it will work without blowing something up?
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: Charging batteries from an alternator
    cptdondo wrote: »
    Thanks. This is in a vehicle, and the batteries charge off the engine alternator, so I'm not at liberty of shutting off the engine when the batteries are charged.

    You could still put in a relay circuit which disconnects the alternator from the batteries based on some way of determining the battery SOC. But that circuit would also have to reconnect the alternator when the battery voltage dropped below a certain level. If you have both a Cranking battery and energy storage batteries, take a look at the discussion of options described at http://www.smartgauge.co.uk/batt_con.html and the rest of the pages linked on that site.
    SMA SB 3000, old BP panels.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #6
    Re: Charging batteries from an alternator
    cptdondo wrote: »
    Thanks. This is in a vehicle, and the batteries charge off the engine alternator, so I'm not at liberty of shutting off the engine when the batteries are charged. I have an automatic charging relay that limits the voltage to between 13.2 and 14.5V, but as long as the voltage is between those limits the alternator is pushing current into the batteries.

    Interesting about the PWM chargers; I have a MorningStar SunSaver Duo hooked up to the batteries, so I could measure the voltage drop across it at 14.4V if I could rig up an appropriate test.

    Any suggestions on the best way to see if it will work without blowing something up?
    On my Boat I have the Alternator feeding a Xantrex C-35 Charge controller. it has no issues, but it not a very smart Alternator. I also feed the same controller with a Battery Charger cranking out 15.5 Volts.
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #7
    Re: Charging batteries from an alternator

    Does anyone know which PWM controllers do not shunt the solar panels when the batteries are charged? That would seem to be the key if I'm going to use one running off an alternator.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #8
    Re: Charging batteries from an alternator

    I didn't think of that being a problem. Here is a marine regulator that is not to expensive, $86. Might end up being a cheap fix. I never hear the Xantrex c-35 shunting anything. It's always pulsing, you can hear it. That bank is 2 GC-2 Batteries and the other one has 4 batteries.

    http://sterling-power-usa.com/proreg-B-12voltadvancedalternatorregulator.aspx
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #9
    Re: Charging batteries from an alternator

    I don't think any "modern" PWM controller "shunts" the solar panels any more (you may find some very old designs still shipping--But they are not from the major solar controller vendors, that I have seen).

    If you use an alternator--There may be some issues.

    First is most alternator have internal voltage regulators set for ~13.8 to 14.2 volts... Putting a PWM regulator in series is not real useful unless the voltage regulator is adjusted for a couple volts higher.

    The second is most DC Alternators are not really designed to hold a set voltage point without a battery bank to buffer/set the average output voltage. Many alternators will go well over 90 VDC if you disconnect their output--Which can damage the input to many PWM charge controllers (and why you should not disconnect the battery from a running car engine--You may blow the ignition/fuel system computers, panel lights, radio, etc. from over voltage.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #10
    Re: Charging batteries from an alternator

    To clarify:

    The system is a typical RV charging system, with a starting battery connected to the alternator. The vehicle also has daytime running lights, so there's a substantial draw even when the starting battery is charged. The regulator in the alternator holds the voltage at a fairly steady 14.4V. It dips at hot idle, but during highway driving it stays at 14.4.

    There is a heavy wire (#2? memory fails me) that goes from the starting battery + to an automatic charging relay. The wire is protected by a 50A breaker on each end. When the voltage from the alternator stabilizes after starting the engine, the ACR closes and connects the deep cycle batteries in parallel with the starting battery. The ACR has a delay in it - something like 30 seconds - where the voltage has to be stable for it to close. It's not an ideal system, but it's typical of an RV installation.

    The alternator will always have the starting battery in the circuit, along with the load of the daytime running lights.

    I'm trying to set up the system so it won't overcharge the deep cycle batteries.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #11
    Re: Charging batteries from an alternator
    cptdondo wrote: »
    I'm trying to set up the system so it won't overcharge the deep cycle batteries.
    If you do not mind losing some charge speed, you could just put one or more Schottky diodes (at ~.2 volts drop each, depending on the current and their power rating) in series with the lead from the relay to the battery.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #12
    Re: Charging batteries from an alternator

    Unless you drive 18 hours per day (who can afford the fuel these days :cry:), I would not install a second battery charge controller between the relay and the deep cycle batteries. It is additional cost/complexity that will not really do anything special (plus, the additional voltage drop would probably prevent you getting anywhere near 14.4 at the battery bank from the vehicle charging system.

    In general, deep cycle batteries need higher charging voltage than vehicle batteries, and since you probably only drive 8 hours a day on long trips (typically for a couple days max), then park--I just would not worry about it.

    If you are still concerned, then I would put an override on the relay (if you can) that allows you to manual shut it down for multi day trips/shore powered nights.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #13
    Re: Charging batteries from an alternator

    How about something like this?

    http://www.mouser.com/ds/2/427/100bgq10-49888.pdf

    That would drop the voltage down to 13.6V, which should be better than 14.4 and lots cheaper than any other solution.

    The lead is protected by a slow-blow 50A thermal breaker and the battery bank has a fast blow 100A fuse on it, so I have to plan for 100A momentary current.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #14
    Re: Charging batteries from an alternator

    I would not worry--14.4 volts for a deep cycle battery is a very comfortable rate of charge (assuming ~75F temperature house battery bank).

    And if your vehicle battery(ies) last for many years--They are not being damaged by over voltage either.

    If you where pumping 14.4 volts into a storage battery for weeks/months at a time--Yea, then you would want to do some sort of float voltage setup (I even use a lamp timer that chargers my stored batteries for just ~1 hour per night).

    I have used the 1 amp battery trickle chargers (designed for battery storage), and they can "boil" a battery dry in a few months of 24x7 usage (even though the batteries are "maintenance free"). Those are the ones I put the 1 hour a day lamp timer on to stop the "over charging" for vehicles that are not used much.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
    Re: Charging batteries from an alternator

    The problem is not the 14.4 Volts which the deep cycle will take (it may even be a bit low for some) but the fact the alternator holds it there all the time. A full battery is not going to like being held at Absorb Voltage for hours on end if you go on a long drive (typical for an RV application). It won't blow up, but its lifespan will be shortened.

    You may have enough V-drop in the wiring to eliminate the problem. Lose about 0.6 Volts and it would be "perfect". Inetdog's idea of the diode is a good one, if you can find a diode that would handle the current (multiple parallel diodes can do it, but may fail in cascade fashion). You could even put a relay in to bypass them for full charging, then let it drop out to reduce the Voltage. Automating it (if that's important to you) might be tricky.

    In the end, check the Voltage at the (fully charged) 'house' battery while the motor is running (above idle) and see what it really is there. It probably is not a problem.
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #16
    Re: Charging batteries from an alternator
    Cariboocoot wrote: »
    The problem is not the 14.4 Volts which the deep cycle will take (it may even be a bit low for some) but the fact the alternator holds it there all the time. A full battery is not going to like being held at Absorb Voltage for hours on end if you go on a long drive (typical for an RV application). It won't blow up, but its lifespan will be shortened.

    My T145s got cooked in just over 3 years. Not bad but certainly not great.
    Cariboocoot wrote: »
    You may have enough V-drop in the wiring to eliminate the problem.

    I sort of screwed myself there; the RV came with a diode isolator which had about a 1v drop. It failed rather spectacularly (as in, total power failure in the middle lane of a busy highway) so I replaced it with a relay which has no voltage drop. The voltage at the batteries is right at 14.4V.
    Cariboocoot wrote: »
    Lose about 0.6 Volts and it would be "perfect". Inetdog's idea of the diode is a good one, if you can find a diode that would handle the current (multiple parallel diodes can do it, but may fail in cascade fashion).

    I did find a 100A diode that would drop that but at no load it drops .4 V and at full load it drops almost 1V - sort of backward from what I want.....
    Cariboocoot wrote: »
    You could even put a relay in to bypass them for full charging, then let it drop out to reduce the Voltage. Automating it (if that's important to you) might be tricky.

    I thought about that. I'm building a home-brew SOC meter based on both voltage and current and temp, so I'll be able to monitor the state pretty accurately. I'm thinking that as the battery gets charged, current flow will go down and battery temp will go up, so that's when I can drop the relay out.

    Fun stuff to do in between working for a living.
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