Feeding both lithium batteries and can battery from charge controller

Hi all,

I’m wondering if it’s possible to feed both the my lithium batteries (with a BMS from Daly) and my van battery from my charge controller (a 40amp one from Renogy). Currently, I have an input from Solar (300 W Renogy panel) and then hookup to lithium batteries and output to 12v sources via a Blue Sea panel. I get that sometimes people do the opposite, where they glean power from the alternator,  but in my case, I’m not using much power and doing a lot of city driving, so my van battery has been dying a lot.

Thanks much,



  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    Lithium and lead acid batteries are just too different to safely parallel and charge...

    You can get DC to DC battery chargers:

    https://www.solar-electric.com/victron-energy-orion-tr-smart-12-12-30a-dc-dc-charger.html (high end w/ Bluetooth)
    https://www.solar-electric.com/xantrex-82-0123-01-echo-charge.html (less expensive, simple)

    I do have a question... If you do a lot of city driving and your van battery has been dying a lot...

    Is the van battery going bad? After 3-5 years and some cycling (lights, radio, laptop, etc.) loads--Starting batteries just do not last very long. If it is the same battery failing--I have seen that a lot... Once an older car type (SLI-starting, lighting, ignition) type lead acid goes dead once (say lights left on overnight)--That battery may seem good, but days or even a couple weeks later will just stop starting the motor. You can jump/charge and it will work again for a few days/weeks, then (without warning--I.e., even parked for two hours, no loads, and restarting) fail to start.

    Do you have a an alternator/charging problem? Check the battery voltage while running (lights on, lights off, etc.). For older (simpler) cars and vans, charging is usually somewhere around 14.0 to 14.4 volts... If it is below 13.8 volts, not enough voltage... New cars with integrated electronics/battery systems/etc... You have to check the manual/online...

    In some cases, folks have had pretty good luck with replacing the plain vanilla car battery with some sort of deep cycle battery (not "marine", real deep cycle) for work vehicles (need to run tools, charge tool batteries, run laptops and exterior lighting, etc.). For lead acid, the deep cycle battery should be at least 20% larger than the standard SLI battery. AGM deep cycle batteries have worked well too.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 19 ✭✭
    Thanks, Bill, for your thorough response!

    Yeah, rather than parallel...I was thinking of some kind of mechanism that would splice from the charge controller.

    If one were to get a separate charge controller, I'd assume that you would need a separate solar panel as well?

    In response to your other questions: hard to tell if the battery is going bad...my understanding is that it's not great to drive only three or so miles in one go because the battery never fully charges. I would also imagine that it takes a bit more heft to start-up a Sprinter Van than a regular car (you'd probably know better than I). I've only had the van for a year and a half, so it's hard to say the behavior of the previous owner.

    So in a way, the battery mimics a bit what you write, but I don't think it's dead (nor do I think it has a charging problem), since the other day I had to drive to get some stuff (drove ~8 miles on the highway each way) and it started fine for 6 times after that, only driving from a couple different locations separated by a few miles.

    Yeah, so just to be clear, I have the lithium for everything I'd be charging, just need the van battery to start the engine and keep the battery charged (and keep the battery healthy). 

    So if there's no easy way/cheap way to do that (can't just create a makeshift 2nd port on the charge controller that runs to the van battery)...what about those ~$50 7watt panels that Harbor Freight sells. You can apparently just plug into the dc outlet to charge up. It does say on the back (in very tiny writing) to use an appropriate charge controller, but if the wattage is that small, what would you need/could you get by without a charge controller. Big takeaway--looking to keep the battery healthy to decrease maintenance costs and startup when I need it to startup.

    Thanks much!

  • mike95490mike95490 Solar Expert Posts: 9,295 ✭✭✭✭✭
    The best thing about the HF panels, is the cardboard box they come in.  

    Seriously though, a 50W panel is not going to do much to recharge the van starting battery.  Your engine & alternator will, in 10 minutes at idle, out produce what the 50w panel could do all day.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • JadesJades Registered Users Posts: 19 ✭✭
    Hahaha...lol. I’ll definitely be taking that back then...though if it’s merely a matter of time, that’s not such a big issue for me...might be sitting 3-days before I start it again...but if it’s just crap that won’t work...then I’ll def return.

    I did take a look at the Xantrex echo charger and it’s reviews...some say it’s the one good thing they do make. It isn’t entirely clear how it works—are you first charging the lithium batteries then transferring that power to the starter battery (would seem like a waste and might as well plug a battery charger into the inverter) or is it taking it straight from the 1st charge controller.

  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    Without knowing much about the van chassis loads... Just how to estimate a system:
    • 100 AH battery * 12 volts * 10% average discharge per day = 120 Watt*hours per day
    • 120 WH per day * 1/0.52 average * 1/3 hours per day average non-winter sun (need your location) = 77 Watt panel minimum
    • 100 AH * 14.2 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge (5% to 13% rate of charge) = 184 Watt array
    So somewhere between 77 and 184+ Watt array + charge controller would probably be a big help for the van battery. Again, not knowing your daily loads/deficit charging/location/etc.--The above is just a SWAG/rough idea of what could be justified.

    You are correct... A Lead Acid battery needs "time on charge" to fully recharge. If you are running 6 minutes, engine off, engine start, another 6 minutes... That is not going to be very good for the van battery.

    If you got 2-3 years from your battery in those driving conditions, you probably are doing well.

    You could even run a 120 VAC 8-10 amp charger from your Lithium system on the days you are driving (use alternator to charge your lithium bank now?) and/or on the "day after" to bring the van battery to "full charge".

    Problem is that "car batteries" are not designed to be deeply cycled (usually around 15% maximum discharge). If you cycle more deeply than that--They just do not have much overall useful life (i.e., the 2-3 years).

    And, if your battery is failing to start after 6-10 stops--It is probably time to change it anyway. It is never going to be "reliable" again.

    I guess this is a "new problem" (first year of operation, the van+battery did OK)? How are you starting the van when it fails (jumper pack, jump from Lithium bank, ???)?

    As Mike says--Stay away from Harbor Freight solar panels. They will not last more than a year or so (if that), and they are "low efficiency" panels (2x larger than standard crystalline solar panels).

    And put a volt meter on the battery terminals and double check you are getting something like 14.0 to 14.4 volts charging (both cold when first started, and after driving 8 miles/hot). Trust but verify.

    If you drive at night--I will start with headlights on once in a while... A very quick idea of battery health to watch the dimming (works well with standard filament head lamps--Not sure with modern LED lights). Or get a volt meter to plug into the cigarette lighter socket (or wire in a "real" panel meter):


    Also--Short start and stop cycles are very hard on any engine... You might want to get your engine oil analyzed and see if you need oil changes more often (engine does not get hot enough/run long enough, water can condense and remain in the oil):

    https://apexoillab.com/component-type/diesel-engines/ (just random site--I know nothing about this site)
    https://diamondbackdiesel.com/product/diesel-engine-oil-analysis/ ($30 per sample?)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    edited June 7 #7
    The idea of the DC to DC charger... It only charges the "other battery" if the first battery is being charged (for example, >13.0 volts). I.e., automatic operation.

    Yes, you could easily get a simple 120 VAC to 12 VDC AC charger--You would just need to manually turn off the charger/AC inverter when you are done (watch the charging current, when it fall to less than 1 amps, your van battery is pretty much charged).

    The AC charger is certainly a cheap/easy solution assuming you have "enough" Lithium/Solar energy to spare.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 19 ✭✭
    Thanks for this! Yes I should look into oil analysis...or just change the oil more often.

    One weird thing is that if I plug a 1.5amp charger...it says charged comparatively fast, though obviously it isn’t charged, so it might be that the battery is shot. I had to jump it again today, so I’ve kind of had it...I think agm is probably the way to go...what do you think of something like this: Check out this item on OfferUp. https://offerup.co/KOGahIDTZgb

    I checked the battery after starting and it was around 13.85...after running for a bit it was about 13.95.

    Will def return the harbor freight panel.

    If you were to just stop and start your car...I thought it was normal to be able to get ~6-10 starts.

  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    Concorde is a high end AGM battery mfg... If that is around an 80 AH @ 12 volt battery--I guess you are looking at almost $300 new (purely a guess--I am not in the business). They have other model line that is designed for solar (lots of deep cycling)--But I am sure this one would work well for you too.

    You can read the battery mfg. manuals here:


    Regarding the quality/worth of this particular battery--You will have to make that decision.
    Me, I always measure the battery box and see if I can cram in a larger AH battery--Even in my daily driver cars/vans/etc.. When we were younger--Taking our kids to swimming and such--Spent a lot of time in the car waiting (reading with dome light, radio, running laptop, etc.)... Did not bother with deep cycle--But the larger battery did do well for me.

    If you check the Lifeline manual, you need around 14.3 Volts +/- 0.1 volts for full charging at 77F (lead acid batteries are very temperature sensitive for proper charging voltage). At that voltage, the battery is fully charged when current falls to 0.5% of AH capacity (i.e., 100 AH = 0.5 amps or less).

    Your van's charging voltage at 13.95 volts sounds a bit low (14.0 to 14.4 volts has been typical for vehicle battery charging)--But with today's modern electronics (and variations in MB Van's hardware over the years)--I have no clue what is "normal" for MB (one forum suggests 14.1 volts--But there are many changes to alternator systems and van models over the years).

    You are correct--Charging with a small 1.5 amp charger and quickly battery going to 100% State of Charge--That is usually the sign of a lead acid battery at end of life--Sufation (battery lead/sulfate turning to black/hard crystals and no longer being part of the charge/discharge chemistry) and other internal factors--The battery will lose Amp*Hour capacity.

    Yes--I would expect >>10 starts in city traffic/delivery/etc. use... Just to get an idea of the battery load... My old diesel Rabbit used to take 50 amps for 30 seconds (1/120th of an hour) to and 5 seconds to crank @ 100 amps (SWAG) to start on a cold day (or something like that--been many decades).
    • 50 Amps * 1/120 of an hour = 0.4 AH
    • 100 Amps * 1/720 (5 seconds to crank) = 0.2 (less really) AH
    • or ~0.6 AH per start
    • 80 AH battery * 0.15 "normal" cycling = 12 AH for normal SLI battery cycling
    • 12 AH / 0.6 AH per start = 20 "cold starts" for normal low stress SLI battery cycling if no charging between starts
    The above is more of a set of guesses... In San Diego, you probably are not cold starting (glow plug cycle) every time you start... And I did not crank my Rabbit for even 5 seconds.

    And if the alternator is capable of >40 amps charging after the start (which it should be):
    • 0.4 AH / 40 AH charging = 0.01 hour of "charging" = 0.6 minutes to "recover" lost starting energy
    Lead acid batteries will accept higher charging current if they are discharged below ~90-80% SoC... So even if you were daily deliveries (like a UPS or FedEx truck)--You should hit a floor where even a minute drive per start cycle--Your battery should not go dead...

    Obviously, this short cycle of discharging and charging is not "great" for battery life--But it does work. Your battery is (most likely) simply worn out at this point... 

    Eventually, you will probably want to get an AC+DC 0 Clamp DMM (digital multi-meter). Besides normal volt/resistance/etc. DMM functions, they have a "clamp" that you clip over a cable, and you can measure current... There are AC+DC clamp meters, and AC only clamp meters... For batteries/solar equipment, you want the AC+DC current clamp meter.

    https://www.amazon.com/gp/product/B019CY4FB4 (mid range AC+DC current clamp meter)

    Using a clamp meter--You can measure the actual charging current and overall system operation. For example, measuring charging voltage and current of the "new battery" at idle and see what it takes--And years down the road check a questionable battery charging current and voltage to see if the charging system or battery is "at fault".

    You can check battery capacity--If the battery is not starting the vehicle... It is probably well under 50% state of charge (use hydrometer or volt meter to measure resting voltage and lookup on charge SoC)... A 1.5 Amp simple charger would charge:
    • 80 AH * 0.50 SoC = 40 AH depth of discharge
    • 40 AH / 1.5 Amps = 26.7 Hours to fully recharge
    If your battery is "recharging" quicker--It is probably sulfated.

    Note: I had a cheap 1 AH charger from the local auto parts store.. My van battery went dead (if the sliding door was open, the computer would drain the battery dead in 1 week or so on occasion). Used the 1 Amp charger to charge to get van started and moved (was second car and did not use much once kids grew up). The charger did charge the van battery (I was moving to replace with new battery)--But the cheap 1 amp charger overheated and literally split the case open. Cheap is cheap... Just watch out that you don't accidentally start a fire with the cheap chargers.

    Anyways... Lots of typing for a "worn out" battery. Replace it. If you want longer life and you do some days with lots of stop and go driving--You can put a simple AC charger or Solar Panel+charge controller on the van (or other charging method) to float the battery for longer life... I have been very happy with the BatteryMinder Duracell brands of 120 VAC float chargers:


    Have a couple vehicles (and my in-laws don't drive much anymore)--And these float chargers have keep the batteries happy even with some loads (Lojack and GPS system in one car, the "computers" in modern vehicles that seem to take a bit of extra energy and for car alarms, radio clocks, etc).

    The "cheap float chargers" -- They always ended up with too high of float voltage and eventually boiled the batteries dry--Cheap charger and new batteries. The "better" chargers--No problems at all.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 19 ✭✭
    “ Anyways... Lots of typing for a "worn out" battery. Replace it.” Really driving home the point though:)

    Thanks, Bill, you’re a wealth of information. This is all good to know. Need to learn more about batteries in general and will look into a clamp meter (I have a pretty run-of-the-cheap mill voltmeter now and have been told I need a better one that measures to the thousandth, so probably with just going with a clamp one.

    Have a Costco membership, so I could get the Duracell, if you think it’s a good one. The current one I have actually is more expensive: it’s this guy— https://www.amazon.com/Battery-Tender-021-0128-Maintain-Damaging/dp/B00068XCQU. But if you think the Duracell is better, I can go with that.

    I do have a space where I could park it and charge it roughly once per week. Not the most convenient. Could I hook up an AGM to Lithium or am I still in the same boat where the above applies?

    Thanks again!

  • JadesJades Registered Users Posts: 19 ✭✭
    Also, apparently this person also has this battery: https://www.sumppumpsdirect.com/Interstate-Batteries-DCM0100/p50877.html which she is willing to sell (Interstate deep cycle 100amp/hours). Thoughts on whether one is better than the other (this clearly has more amp/hours)?
  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    The Battery Tender--I have not used it--But from what I have read, it is also a very good float charger.

    You can also take your volt meter and measure the charging voltage... ~14.0 to 14.4 is good for a "general" battery float charging a discharged battery. 13.6 to 13.8 volts--Not really charging very fast--And a bit high for a long term "float charger". However, if you plug the van in for a few days and disconnect--That will not hurt anything.

    And when in float (after battery is full), you want to see around 13.3 to 13.6 volts or so (higher voltage, can cause battery to "gas" and lose water).

    Interstate batteries are OK (from what I have read)--There is an issue that the "big brands" are, many times, built by some other big Mfg. and then private labeled for retail sale... And what was made 5 years ago--May be a completely different mfg. today. For example (don't know anything about site):


    Interstate Batteries are one of the most popular battery brands. They are reliable and relatively affordable. A common question many have is who makes them. Who makes Interstate Batteries?

    The maker of Interstate Batteries is Brookfield Business Partners and Exide Technologies. The Interstate Battery System of America, Inc., a privately owned battery marketing and distribution company in the United States, markets these batteries.

    There was a previous report that Johnson Controls is making 65% of Interstate Batteries. However, this is no longer true. Johnson Controls is not currently making these batteries. The company Brookfield and Exide Technologies has already taken over this job.  

    Read on to learn more about who makes Interstate batteries, the different types of Interstate batteries, and their lifespan...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,769 admin
    You can use the Battery Tender on your AC inverter... it will not draw much power (1.2 amps * 14.0 volts = 16.8 watts) or less than 30 Watts with all the losses.
    You can look the lithium bank voltage vs the AGM voltages... probably Will charge ok. But long term float voltage is too high (days/weeks/months).
    One thing to watch out for, lithium batteries are great batteries. If you connect a li ion battery directly to a partially charged AGM Li Ion battery with heavy cables, you can get very high current flow and damage one or both batteries.
    That is why a magic black box DC to DC battery charger is usually recommended. The charger will only draw rated current and not damage either battery.
    Longer smaller AWG cable can add enough resistance to limit peak current flow... However, i don't have enough information to do the math at the moment.
    Details matter, again....
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 19 ✭✭
    If you’re buying a used battery is there a way to test it with a multimeter, or do you have to take it to an auto parts store to be tested? I saw a battery load on the multimeter and there are some YouTube’s about this, not sure whether it works.

    Let’s say that a battery has a 13.2 voltage charged but fails for 870 cold cranking amps, is that just a bad battery, or is that something that would be fine for solar just not good with starting vehicles?

  • mike95490mike95490 Solar Expert Posts: 9,295 ✭✭✭✭✭
    An automotive cranking battery is not suitable for solar/off-grid use.  It will work for a couple weeks, then start giving your trouble.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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