Battery Scenario...Is this feasible?

Wanderman
Wanderman Solar Expert Posts: 180 ✭✭✭
in Off Grid Solar & Battery Systems #1
Been working through some electrical oddities with the Morningstar TS-45-MPPT controller installed in my system onboard an RV.

One of the problems is an odd voltage reading at the MPPT when the engine is started (this is covered in another thread.)

First question:

I have 2 existing Trojan 27TMX batteries for the "house section" and 1 group 24 battery for the "Engine" battery. Would it be possible to remove the group 24 (non deep cycle) and add 1 (or 2, since I have the space) more Trojan 27TMXs. This would necessitate starting the main engine from the 3 (or 4) Trojans. I do not know the exact instantaneous draw for the starter, but it starts within 3-5 seconds. Would spreading this draw across 3 (or 4) Deep cycles be acceptable?

I have 2/0 cabling running from the "Engine" battery to the main 12V distribution box/switch and the run is around 6 feet max (perhaps a bit less)

If I added 2 batteries I and left them tied together, I would, theoretically, be able to add around 210 aH (105-ish) usable to the system. I understand the risk of running the system down too low to start the engine, but I have a jump box AND low voltage alarms set to avoid that. I also have yet to see anything below 70%+ with use.

Comments?

Rich
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Battery Scenario...Is this feasible?

    Depending on what engine/starter, the draw will be in the neighbourhood of 60 Amps +/-. Even one deep cycle will handle this. On an "equivalent" basis, most auto batteries aren't much more than 100 Amp hours. The concern would be V-drop over the wiring distance, even with heavy cables. 2/0 over 6 feet ought to handle it. But I suspect this won't cure the interference problem between the alternator and TriStar.

    In my opinion it is best to keep the starting and "house" systems entirely separate, and only connect them if/when necessary with a manual connection.
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #3
    Re: Battery Scenario...Is this feasible?

    Cariboocoot,
    As usual you are speedy and concise! I will still have the ability to split the systems at the Perko switch (OFF/1/ALL/2) but this will really help since I am entirely out of space fro additional batteries. I've always planned for 4 of them and only had room for 2. I am working on a 400aH Lithium Phosphate solution, but the cost and time from the manufacturer are still prohibitive.

    I spoke with Morningstar about the voltage issue. They told me that"

    >> It is possible that the engine charge current, in addition to the MPPT
    >> bulk charge current cause the battery voltage rise extremely fast. In
    >> this instance, the MPPT woult take a few minutes to begin limiting
    >> current to the battery.
    "

    The max looks to be 16.72 volts. I will be running some experiments Thursday and Friday to determine if it ever rises above that value and indeed, if it ever returns to normal levels. If it does, I can program the MPPT ontroller to ignore the HVD
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #4
    Re: Battery Scenario...Is this feasible?

    Cariboocoot,

    How about setting it up so the 2 house batteries are charged first and then the engine/extra battery is charged second with discharge/use the same way. I discharge the 1st set (house) then at a particular point it will switch to the (engine/extra) Opposite when the engine is charging. This way if I need them, they are there.

    Seems like some kind of battery isolator (FET!)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Battery Scenario...Is this feasible?

    I see some clues here. From what Morningstar said and the symptoms displayed sounds similar to other problems reported when using a PWM controller in parallel with an MPPT; the pulsing of the PWM sometimes confuses the MPPT. An alternator should be a linear Voltage source. Gone are the days when "Voltage regulators" in vehicles were nothing but cut-outs that really did make for some odd Voltages. But just because it should be linear does not mean that it is.

    Another point; MS controllers have Voltage sensing leads. If not in use they will determine Voltage based on what is found on the output leads. Here you have multiple potential problems with getting the Voltage right: remote temp sensor (if equipped) can only be on one battery, Voltage sense wires can only be on one battery, difference in wiring between the two batteries would cause any form of Voltage detection to vary.

    16.72 Volts is definitely too high for anything.

    I don't remember MS controllers having a programmable AUX function, which would be a good way of switching the solar output over (main bank reaches Float, AUX activates relay to send charge to second bank).

    Let's look at your goal here. You want the solar to be able to keep the second (starting) battery up so it is ready to go? You want to be able to use the second battery for additional inverter power when parked? (I'd steer clear of that option.) You want the vehicle alternator to be able to boost the "house" battery when on the go?

    Considering the inevitable differences between the two battery banks (slight resistance in wiring on a 12 Volt system is actually a lot of difference) there's no chance the charging/Voltages can be the same in both even if they were identical banks.

    Keeping the two systems isolated and evaluating the individual performance would be the first step. as it is, it sounds like they won't play together well unless a second charge controller is used to connect the two upon charging. That's not a really desirable option.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Battery Scenario...Is this feasible?

    But having DC to DC battery chargers is a fairly common solution for house/vehicle battery solutions. It does reduce some of the isolation issues.

    One possible solution is this Xantrex battery to battery charger.

    There are a lot more than just this one--But you might need to check an RV forum/supplier for more options.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #7
    Re: Battery Scenario...Is this feasible?

    BB,
    I actually have a New In The Box Trik-L-Start that I ordered to make sure the old style engine battery stayed charged. I believe I can come up with a system whereby I can have the best of both worlds and eliminate the traditional starting battery from the mix. AND get rid of the extraneous switch.

    Rich
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #8
    Re: Battery Scenario...Is this feasible?

    WARNING!
    I just received a comment on another site that purports to be from a PHD Aerospace Engineer. While I cannot, for obvious reasons confirm this. He maintains that there is no way that even multiple bridges Deep Cycle batteries could survive multiple engine starts.

    This just seems off to me. I agree that spreading a 60-100 amp 3-5 second load across 4 Trojan 27TMX batteries (or even 3!) should have a momentary draw lower than the C rating for discharge.

    Am I missing something this guy knows???

    Seems counter-intuitive.

    Rich
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Battery Scenario...Is this feasible?

    He should stick to aeroplanes. :roll:

    While deep cycles are not ideal for starting (not designed for supplying the sudden heavy current demands of starting motors - different plate design; not as much area) they are basically the same lead-acid technology. Also the charging profiles are different.

    As with any battery you get the best performance matching type with application. You can use automotive batteries for RE, but they don't perform well or last long. Vice-versa is also true.
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #10
    Re: Battery Scenario...Is this feasible?

    That's what I figured. Are we talking about early death syndrome for the Trojans or just an odd use, but no harm.

    Thanks, CC

    Rich
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #11
    Re: Battery Scenario...Is this feasible?
    Wanderman wrote: »
    That's what I figured. Are we talking about early death syndrome for the Trojans or just an odd use, but no harm.

    Thanks, CC

    Rich

    As long as you do not go too deep into discharge (which engine starting should not do unless you flood the engine and just keep on cranking trying to start) and do not drive the temperature of the battery too high, you should not have any long term bad effect.
    You are right though that for a large engine which needs a lot of cranking amps to turn the engine over, you will need to parallel several RE batteries to get the same current capability of a cranking battery of the same size.
    I once had a very old Datsun pickup with a 6 volt electrical system, and in the winter the starter pulled so much current that although the engine cranked just fine the terminal voltage of the cold battery dropped so low that the ignition would not drive a spark to actually start the engine. The solution was to let the battery run the ignition and turn over the engine with a hand crank.
    SMA SB 3000, old BP panels.
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  • Jim45D
    Jim45D Solar Expert Posts: 102 ✭✭
    #12
    Re: Battery Scenario...Is this feasible?
    Cariboocoot wrote: »
    Depending on what engine/starter, the draw will be in the neighbourhood of 60 Amps +/-. Even one deep cycle will handle this. On an "equivalent" basis, most auto batteries aren't much more than 100 Amp hours. The concern would be V-drop over the wiring distance, even with heavy cables. 2/0 over 6 feet ought to handle it. But I suspect this won't cure the interference problem between the alternator and TriStar.

    In my opinion it is best to keep the starting and "house" systems entirely separate, and only connect them if/when necessary with a manual connection.

    This happened to me when my mechanic forgot to move the selector switch from house to start. As Coot says, "It depends on the engine/starter." My engine is in the 400 CI range. The house batteries started it just fine, but in the process it wreaked holy havoc with my settings on the Heart Interface Inverter, the instrumentation, etc. The instrumentation is the Link 10. It say's plainly in the manuel to keep the house and starting batteries seperated. I didn't overlook that, my mechanic did. Since that time, I ran completely different wiring to the alternator and starter to JUST the starting battery. The selector switch has since been disconnected. I installed a 10 watt panel solely for the starter battery.

    I don't remember the name of the thread, but it was sometime in March, a year ago this past when I started asking questions here on this forum regarding what to do. Coot, and someone else advised me as to what to try....it worked. The entire metering on the MPPT, and the inverter were all amuck, and it stayed as such. It would be best to follow what Coot has already said above.......KEEP starting and House seperate. You'll fare well.
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #13
    Re: Battery Scenario...Is this feasible?

    Interesting. I have a 200 (really 250-ish) AMP shunt that feeds my battery monitor. I'm going to take a look at the draw with it and a clamp on dc ammeter today. I'll wager the loads is lower than 100 amps...I would have noticed that draw on the big display when i started the engine. I did not see any ill effects on metering at all. My inverter will probably pull more running the microwave than the start does!

    Rich
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #14
    Re: Battery Scenario...Is this feasible?

    I have a 4 hp @ 24 V bow thruster that pulls at least 250 amps ( the rating is for 320 amps ) and a set 4 GC'2s and has done so for 15 years with any effect other than the batteries only last 5-6 years. The same batteries have a TR-1524 inverter hooked to them without any issue. If I pull them below 22 Volts it will cut out the inverter with that heavy of a amp draw them. Granted that inverter is only being used as a charger and a power source for a 5000 btu ac. If I had a 100-200 amp shunt connected to them , I guess I would be concerned.
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #15
    Re: Battery Scenario...Is this feasible?

    Astounding! I just tested the draw from my starter.

    According to the BM-2 running through the shunt. With the engine start battery OUT of the loop (disconnected at the terminal) the overall draw maxed at 35 AMPs. Now I had 3 amps coming in from the Solar Panels (raining here) and likely another 3 from the onboard converter charger. The so gross seems to be around 41 AMP. Let's call it 50 total. If I add 1 additional Trojan 27TMX (where am I going to find a used one?) that would mean 50/3 +(net wire loss) = 20 amps per battery (more or less) According to Trojan the 27TMX:


    Capacity Minutes Cranking Performance 5 Hr RateAH 20 Hr Rate AH Voltage Terminal
    @25 Amps @56 Amps @75 Amps CCA @0°F CCA @32°F
    175 - 45 530 650 85 105 12 WNT, LT

    So this means it's way UNDER the recommended max draws.

    Can't argue with the factory ratings, right.

    I really like this solution. It actually simplifies the overall system. I could put a voltage battery disconnect in place of the manual switch on the engine battery...or better still one of the remote ones like a Battery Brain. Just have to figure out which one. It should kick in except in a catastrophe discharge situation. So far I haven't gotten that low at all.

    What do you all think?

    Rich
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #16
    Re: Battery Scenario...Is this feasible?

    Sounds a bit low for starting current (unless this is a fairly small engine). Also, being from New Yawk--You know the cold weather starting issues (cold oil, hard starting).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    #17
    Re: Battery Scenario...Is this feasible?

    BB,
    I really don't use the RV in the winter. Coldest I would even THINK about going out would be well above freezing. I LIKE having the water system working! It is stored when it gets cold.

    Even if we double the draw, it's still well within the manufacturer specs for a draw. I will change out the engine battery for another 27 TMX (maybe put another one in as well.) Now all I have to do is find 1 or 2 gently used Trojan 27 TMXs that I can put in here. The other ones are fine, but a few years old, so new ones would be a waste.

    You know, I LIKE puzzling this stuff out!

    Rich
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