Hybrid Mongrel Battery

Why do we have a Hybrid Mongrel Battery in our RV?

We are creating a hybrid mongrel battery for our RV. The system will have lots of solar, and both lead-acid and lithium batteries. We are going to load up the RV with lots of lithium batteries and eventually put as much solar on the roof as possible. By the end of the week, the rough fit for the lithium will be finished establishing all the locations for the wire runs and layout. After that, I will start making some seriously large wires.

We are staying in Boise and last week we made arrangements for our battery overhaul here in Boise.  This week we take delivery of all the components. We have family here in Boise and they can accept deliveries of all the stuff that we will need for our installation. Parts and some specialty tools are starting to arrive daily. Most of the stuff will be here before the end of the week. 

Our Hybrid Mongrel Battery Overview

12 volt, 600 amp-hours lead-acid AGM, and 800 amp-hours of lithium. Soon we will add 1800 – 2400 watts of solar.

Battery types

The two battery types we have and will install are lead-acid AGM (absorbed glass mat) which is an advanced type of sealed (meaning that there is no need to add water to make the battery work) lead-acid battery. We are going to add lithium iron phosphate batteries (which don’t use lead to store the energy) to our system to increase battery capacity. For a complete description of batteries, we have laid out all the details on our blog at Solar Series Part Three. Look at the posts titled Understanding Batteries and Lithium / Lead-Acid Final Analysis.

Feeding the big refrigerator

We knew that our new, larger, and nicer RV was going to be an energy hog before we got it.  We discovered how hungry it was while we were moving from RV to RV. Our AGM batteries needed recharging every six hours to make sure we didn’t pull the voltage down too much and start battery damage. I have even turned the refrigerator off overnight to prevent battery damage. Turning the refrigerator off allows me to have enough power to run the remainder of the RV DC house electrical system overnight. 

We also had problems with our engine start battery (chassis) a couple of weeks back. That problem, which is solved, has nothing to do with our house battery system makeover. 

Our Hybrid Mongrel Battery

Until now, the words “replace” and “convert” has dominated my thinking about my lithium battery install. Now the word “combine” is more dominant in my thinking. By combining the two different kinds of batteries I will be creating a hybrid battery system. Or put a different way perhaps I’m creating a hybrid mongrel battery — instead of a purebred.

Lead-acid and lithium batteries need to be on separate systems!!!

Everyone I have talked to has said not to mix lead-acid batteries with lithium batteries for very good reasons.  First and most important, the charging profile of lead-acid batteries is not similar to lithium. If you charge lead-acid the same as lithium you will destroy them. If you charge lithium at the same as lead-acid you will never get anything even close to a full charge on the lithium.

Instead of replacing functional but insufficient lead-acid AGM batteries, I am working on dividing my house system into two parts. The lead-acid AGMs will power the DC house and the lithium will power the AC part of the house. Dividing the system, both in charging and dividing the items each system powers is the key to making my RV’s hybrid mongrel battery system work.

I have been corresponding with some amazing friends about the idea and there haven’t been any serious objections and it seems to be a functional idea. There may be some really good reasons that I shouldn’t try to set the battery system up this way but I can’t find any show-stoppers. The most serious insightful critical look at the plan pointed out that it is slightly more complicated than just replacing the lead-acid AGM batteries with lithium.  

Replacing all the AGM batteries with lithium isn’t a reasonable idea due to cost. This is the reason we are creating a hybrid mongrel battery system. Some say that 800 amps of lithium are enough for anyone. I reply, more is way better.

We are not saving the AGM batteries because I think the lithium batteries may be insufficient. Really I am saving the fully functional AGM batteries from the landfill.

I have never heard of anyone making a hybrid mongrel battery and maybe there is a really good reason that I don’t want to do it this way, but I don’t see why not. 

Another Factor

In our RV the factory-installed AGM batteries are located about 15 feet away from the inverter. Thus anytime the AGM batteries powered the inverter there was the potential for a huge voltage drop between the batteries and the inverter. With my envisioned setup, the lithium batteries will be very close to the inverter and thus not have the extra burden of unnecessary voltage drop. This will work much better than my AGM batteries ever could. My inverter will work better and of course, the AGM batteries won’t have to do very much work.

Hybrid Mongrel Battery would break down like this: 

Lead-Acid AGM:

12v = All DC house loads (lights, etc.), generator start, and engine start assist.

The only change to the lead-acid AGM batteries is that they are currently connected to the inverter/converter. Instead, they will be connected to a dedicated charger.  The AGMs will still be charged from the engine alternator when we drive. Since they will already be full, nearly all the time, even the engine alternator won’t be charging them very much. 

Since the AGMs will not be connected to the inverter/charger, they will not be charged by running the generator or from shore power directly. Instead, the generator (or shore power) will produce the AC current and then from there, the AGMs will be charged from a dedicated battery charger. There is a very real loss of efficiency that we will need to measure when charging the AGM batteries this way.


All AC house loads — refrigerator, microwave, instant-pot, computers, and televisions will be powered from the lithium batteries.  We won’t be operating heating or air conditioning unless the battery system is full or nearly full.

Lithium will be recharged via the inverter/converter powered by the generator/shore power or solar. Solar will be dedicated to recharging the lithium battery bank. The engine alternator will not charge the lithium batteries.

Inverter/converter will get power from either the generator or shore power via the transfer switch with no change to that wiring.  When not operating the generator or connected to shore power, the inverter/converter will get power from the lithium and then use the battery power to create the AC power for the house’s AC electrical needs.


  • I do not throw away the 600 amp-hour lead-acid AGM. Thus I jump from 800 usable amps of lithium to nearly 1100 usable amps combined. 
  • The AGM is already installed, no change to that wiring.
  • The AGMs should live a much longer, pampered life.
  • The AGMs will be nearly full most of the time and charged slowly which increases their lifespan.
  • The load on the AGMs is cut by about 80%, even more in the summer. This would be a good reason to eliminate them other than they are already installed and paid for.
  • Since the draw on the AGMs will be so small I should never get in a situation where I cannot run the propane furnace or start the generator — even if the lithium is nearly dead.
  • Even if the lithium charge is really low, the AGM batteries should span several days of normal DC house operations.
  • I don’t change the AC house electrical system except to disconnect the AGM from the inverter/converter.
  • The engine is allowed to recharge the AGM without modification.
  • The alternator is not connected to lithium. This eliminates the risk of too many amps of electricity going from the alternator to the lithium, thus damaging the alternator.
  • If I change RVs then I can take my lithium with me. 
  • If I change RVs then I can take also take the solar with me. The only thing I would have to leave behind would be the mounting feet and wires.


  • I still have to carry 360 pounds of lead.
  • The AGMs will need a dedicated charger.
  • Without generator or shore power, the lithium will discharge to maintain the charge on the AGMs.

How are we going to control the Hybrid Mongrel Battery system?

For the lithium part of the system we are going to use a battery monitor to tell much electricity is put into and taken out of the batteries. Here is a link to our article about our battery monitor. Battery Monitor

For the AGM batteries ideally, I would have a second battery monitor but instead, I am going to try to use the voltmeter I already have. This will save some money. Since the loads on the AGM batteries are very low I don’t anticipate ever departing from the almost fully charged state.

Future development:

  • To restrict the re-charging of the AGM batteries I could use a simple timer. This would restrict the recharging of the AGMs to periods where solar is anticipated. This timer would allow the AGM batteries to decrease their charge and then recharge when power is ample, otherwise, the AGM batteries would always be 100% charged.
  • When the AGM batteries no longer are functional, I would probably replace them with lithium.

Currently, I am making wires and I should know (by testing) early next week if this layout is going to work.

Anyway, this is my current plan, let me know what you think about my Hybrid Mongrel Battery.

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This is what we used to create our RV Hybrid Mongeral Battery system. We do not get commissions on the links and are provided only to help our friends.

Lion Energy Lithium Iron Phosphate Batteries

Battle Born Lithium Iron Phosphate Batteries

Victron BMV-712 Battery Monitor with built-in Bluetooth

Update: June 27, 2020, The battery system of both lead-acid and lithium is working wonderfully. Here is one surprise we found about the lithium battery. It is so easy to recharge. Here is the story. Massive Electric Bucket

18 thoughts on “Why do we have a Hybrid Mongrel Battery in our RV?”

  1. From a technical stand point, the split system should function fine.
    The biggest downside I see is my loss of your old AGM’s now that you are using them.
    Will the AGM charger when on shore or generator power for automatically step down to un-monitored trickle charge or will you have to control it manually?

    1. The first charger for the AGMs will source from AC power and thus be able to charge anytime I have AC power, lithium via the inverter/converter, generator or shore power.

      The second step in recharging the AGMs will be to restrict recharge to times when the solar is anticipated to have already maxed out the lithium — since lithium takes a charge much faster than the AGM. I could do this on voltage, state of charge, or just use a timer.

      This refinement would only function to allow the AGM batteries to take some of the load daily through discharge and then recharge. Otherwise, they would only be held to backup the lithium — if the lithium ever were to be fully discharged.

      Finally, I may spend some money on a DC-DC charger for the AGMs and that would save the power wasted converting DC to AC and then back to DC. This however is merely a refinement, not a huge gain, assuming I have ample power to go around.

  2. My fifth wheel doesn’t have an inverter yet, but we do have two new standard batteries. We recharge them with our generator but it seems to take forever. Would you recommend this set up to add maybe two lithium batteries and create a split system?

    1. First, I think you should wait at least until I get results.

      Our batteries were insufficient to run our refrigerator overnight, that is why we went to lithium in the first place.

      Really our old batteries are now the “backup” to the new batteries. Our lithium is more than two – possibly three times more capable than our old batteries.

      Since you don’t also have an engine to recharge your batteries a straight conversion to lithium would probably be the biggest benefit. We have lots of reasons that this (should) work for us — that may not apply to you. Send me an email and we can chat about it.

  3. Since it seems you don’t have a weight issue, still carrying around the AGMs. Why didn’t you just get more AGM batteries? AGMs are much less than lithium.

    1. First, lithium has a much lower lifetime cost than any lead-acid battery. Check out our posts on that subject at https://foxrvtravel.com/rv-solar-series-batteries-lead-or-lithium/ and
      But for us the number one reason lithium is better is that it is going to make our solar much better. Lead-acid batteries resist charge to the point as to handicap the solar, whereas lithium easily accepts all the energy the panels produce until they are nearly 100% full.

    1. Jeff, probably not, the reason I can do this is that I have lots of storage room and lots of weight carrying capacity.

    1. Chris, On the generator or shore power we should be able to put 125 amps per hour into the 800 amp-hour battery bank until nearly 100% full. On the solar, assuming a good sun angle we will achieve about 100 amps per hour charge rate.

    1. Thumb rules don’t work and it doesn’t matter what size your batteries are.

      But it does matter what kind of batteries you use. Lead-acid batteries (all lead-acid) batteries resist a charge and the last 10% of the charge takes as much or more time than the first 80% of the charge.

      Lithium however acts more like a bucket and does not resist being charged.

      I will be outlining how I sized my solar to my actual usage in the future. I outlined my previous method two years ago. All of this is documented in the solar section. The biggest difference between my new measurement method is I had a good answer in one day, rather than over a year of camping before I put the solar on.

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