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Thursday, August 2, 2012

Shouldn't You Be Able To Use Deep Cycle Batteries As Engine Start Batteries?

My Tiny Battery Drawer
    When I was planning my solar charging system, I had to determine the storage capacity of my house batteries. Since my battery compartment is so small I can only fit 2 Group 27 Deep Cycles in there. That gives me 2 x 105ah which is 210ah total. Of course, since you should only discharge lead acid batteries to 50% I actually only have 105ah to use. This was...OK. Not great, but OK.

I seldom use more than 80ah in a given day/night cycle so I was mostly alright. More would be better! I am looking into the new technology Lithium Phosphate batteries to create much larger storage "tank" for my use. Around 400ah total that you can discharge down to 20% remaining giving me 320ah total! This is NOT ready for prime time. They are REALLY expensive (around $2100 for 400ah).

Battery Master Switch (OFF/1/ALL/2) - On Right
In my RV I have a switch that separates the Engine/Chassis battery from the two House Batteries. Most/All RVs have a similar setup. Some of them can charge the house batteries from the alternator when the engine battery is full, and some will do the reverse. Namely, charging the chassis battery when the house batteries are full. Unfortunately, most deep cycle batteries want to charge at a different voltage than a typical automotive start battery. My deep cycles charge at 14.8V and the Engine battery likes 14.4V If I connect the two systems, I am either undercharging the house batteries or overcharging the engine battery. Now what?


Typical 5 Amp "Power Theft" Device
I came up with a way around one direction of this problem. You have to figure out a way to isolate the systems and charge one from the other at the proper voltage. Easy to do FROM the house batteries TO the engine battery. There are a few devices that will "steal" around 5 amps from a charging house battery system and flow it to the engine battery. Everything is good. What about the reverse. Many, if not all, RVs have a battery isolator that will prevent the flow of electricity from one battery system to the other. This works well. Now it will allow you to charge your house batteries (after your engine battery is charged). Some RVs just tie the battery systems together when the engine is running. Now we're back to the wants and needs of deep cycle batteries vs. engine start batteries. How can I get 14.8V volt charge to the house batteries when my vehicle only puts out 14.4!

MosFET Battery Isolator
That got me thinking, what would happen if I tied all the batteries together and replaced the engine start battery with 1 (or maybe 2!) Deep cycles. I would then have 4 x 105ah giving me a total of 420ah with 210ah usable! The only issue is the starting of the engine. I found out, the average draw from a V8 engine starter is 60amps. If I split this between 3 or 4 batteries I get 20 or less amps. No problem for modern deep cycles!!!


***WARNING!! - I've Been receiving conflicting information about using multiple deep cycles to start an engine. TRY THIS AT YOUR OWN RISK!!!***


After testing the max draw from the two deep cycles in my RV. (I disconnected my engine battery first) I've determined for my 5.9L (360 cu. in) V8 I draw less than 50 Amps! It actually read 35 Amps, but I has some power coming in from the panels ~6amps or so. The manufacturer gives these deep cycles a CCA (Cold Cranking Amp) rating so they CAN be used as start batteries. The <25 Amp, per battery, draw I am using for 3-5 seconds falls well within the specs for these batteries. My inverter can use significantly more than this! I believe we are safe. When I add in the third (or even fourth) battery, the draw per battery goes down even further. If you are nervous, let me test it for the rest of the season and I'll report on the condition of my batteries at that time. Now where am I going to find a used 27 TMX battery or 2????

Now for the down sides. If you leave your battery systems connected together, you could deplete them enough that you couldn't start you main engine and perhaps even your generator. This is bad. I am very mindful of my state of charge, and even have low voltage alarms in place to stop that from happening. I also carry a "jumper box" which is simply a sealed lead acid battery in a plastic box with attached jumper cables. That's a last resort.

I am looking into setting up a way to have the batteries charge 2 at a time. And then, get used 2 at a time. Yes, I could do this with a switch, but I'd like it to be automatic. I found that several charge controllers will provide this, but of course, it's not the one I already have! I know it's all pretty confusing, but there is a good solution. Could this be the stop-gap measure until the Lithium Phosphate batteries are ready for prim-time?

More info on this as I work through the details. Stay Tuned!

Be Seeing You...Down The Road,

Rich "The Wanderman"
www.thewanderman.com



13 comments:

  1. Main reason you can't use Deep Cycles for starting is the construction of the lead used in the two types of batteries.

    In Starting batteries, they use a lead sponge designed for a large burst of high amperage for a short period of time, in deep cycles they used heavy lead plates that are meant for slower amp draws out over a long period of time.

    If you use a starting battery in a deep cycle situation, the heavy extended discharging that normally occurs will cause the lead sponge to break down rapidly and kill the battery.

    The reverse occurs if you use a deep cycle for a starter, the heavy amp discharge will drastically shorten the cycle life of the battery as they're not designed to draw down at a high amperage output rate like a starting battery.

    The compromise is called a hybrid, which are used in marine applications that used a coarser lead sponge that can be draw down like a deep cycle, or start a boat motor.

    The caveat is these hybrid batteries usually have a reduce amp/h capacity and won't last as long as a true deep cycle. They're CCA are also along the line of a smaller marine motor, usually good for starting a smaller motor, but not a larger one as 650 CCA is pretty much the average for alot of Marine Hybrid batteries unless you get into the higher end units that can set you back between $200-300 a battery.

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    Replies
    1. Matt,
      I have to disagree. With a 60 amp start load spread across 4 batteries the momentary draw is very low. A modern deep cycle will handle that load easily.

      I'll be testing it in the coming months.

      Thanks!

      Rich "The Wanderman"

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  2. I also think your starting amperage figure is a bit on the low side for a v8, if 60amps was all that was needed to turn a V8 over, most higher quality dead start chargers wouldn't need a 110 amp or greater output mode for jump starting :).

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    Replies
    1. Matt,
      Even so, figure 25 amps per battery for a few seconds. VERY low. The C ratings are higher than that on a deep cycle.

      Rich "The Wanderman"

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    2. Do you plan to run a new negative feed cable directly from your engine block to the very last battery in your set?

      If you don't what's going to happen is the first deep cycle in the line is going to end up being the one that delivers the brunt of the starting load, which will shorten the service life of the battery.

      If you truly need a better battery capacity, Torklift makes a frame hangable battery box kit now that will allow you to add additional deep cycles along your frame rails without the need to build a new box.

      Why risk damaging $200 deep cycles to start a motorhome when a $70 starting battery does the job fine right now?

      On the subject of that new Li-Ion battery, I think you're going to be waiting long enough to go through a couple sets of regular deep cycles before that becomes reasonable in price.

      Li-Ion technology has been around for decades now and has not really come down in price terribly much because of the cost of the materials to make the battery cell.

      Delete
    3. Matt,
      Actually I am working on a 1500.00 400aH Lithium Phosphate solution that includes a BMS. Should be ready well before next season AND the price comes down every few weeks.

      My Battery wiring will draw equally from all four batteries. The cabling was done originally to add 2 additional Deep cycles to the engine bay. There's a great online diagram of the wiring of multiple batteries to properly split loads. Of course, I cannot for the life of me find the URL right now!

      Murphy WAS an optimist!

      Rich "The Wanderman"

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    4. Have you already altered the block ground that comes from the current starting battery to your engine block so that it is connected to the last negative post of your deep cycle bank?

      If so, then you're definitely ready to try your high dollar experiment :). Be sure to do it middle of the harshest part of winter to get a true test to be sure that it will work as you hope :), so that when you do go fulltiming you won't strand yourself if you get caught up in extended freezing and your batteries get cold.

      ;)

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    5. Matt,
      The ground is switchable now. That was likely setup for an earlier system. Testing won't likely be in winter...too cold!

      Thanks,

      Rich "The Wanderman"

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  3. One last thing, yer battery draw design is wasting alot of space. Junk that piece of garbage and get a good tray, you should be able to fit paired 6 volt trojans or two group 31 SCS225's in there that'd give you 260 AH's.

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    Replies
    1. Matt,
      I can easily fit 2 SC225's in there. I wrote about that in an earlier article. I am waiting until the Lithium Phosphate packs will be ready before the imminent death of the 27TMXs I have now :) The drawer is really the best design for the existing space. I COULD retrofit a platform, but it wouldn't add any significant space for batteries.

      Bummer.

      Rich "The Wanderman"

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  4. I have replaced my old two starter batteries and four house batteries with six new AGM 6V 300 Ah batteries giving me a total of 900 12V Ah. I start the 330 hp Cat 7 engine from the same battery bank. The conversion happened three years ago and I never had a problem starting the engine using the coach full time. I have a Blue Sky Solar Boost 2024i controller which tells me everything about my battery status.

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    Replies
    1. Chris,
      My system, albeit MUCH smaller than yours will be a similar setup. With a single exception I have not had anyone believe this was an issue at all. Trojan even list its deep cycles with a CCA at various temperatures.

      Thanks for chiming in!

      Rich "The Wanderman"

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  5. True thick plate deep cycle batteries do not have a CCA rating. Thin plate marine/rv deep cycle batteries do have CCA rating because they are starting batteries with characteristics of deep cycle. True thick plate deep cycle batteries have about twice the charge life cycles as thin plates marine/rv deep cycles do. Check with Trojan I did got a lot of good information from them. They do make a few 12V thick plate true deep cycle batteries that do not CCA ratings.

    ReplyDelete

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