Dual Batteries with Lithium - a cryptozoological study

[LeishaShannon]

There is no (cheap) way to limit current and your alternator is probably capable of pushing 14v @ 40A+ into the battery. Uncontrolled 1C+ charging is... not great.
40Ah nominal battery so ~20Ah usable? This seems like a lot of added complexity for very little capacity. eg. your starter battery probably has more usable energy than the lithium.
Lithium feels like a solution in search of a problem in this particular case :/

 

[KevinK]

The Valence battery product is interesting in that is uses a chemistry which gives a nominal voltage of 3.2 volts per cell - so with four of them in series it ends up equaling 12.8 VDC - very similar to a lead acid type battery at 12.6 VDC (using six cells in series). That make operating it in parallel with a lead acid battery much simpler.

Other lithium battery chemistries are not as fortunate to have the same voltage - for example, the Nissan Leaf uses a battery that has a nominal voltage of 3.75 volts per cell - so with four in series the total voltage equals 15 VDC - and three in series equals 11.25 VDC - Too high and too low.

Interested to hear how this works out!

Yeah, in talking with them about the battery they mentioned how close they tried to get to lead acid so it would be simple to convert.

There is no (cheap) way to limit current and your alternator is probably capable of pushing 14v @ 40A+ into the battery. Uncontrolled 1C+ charging is... not great.
40Ah nominal battery so ~20Ah usable? This seems like a lot of added complexity for very little capacity. eg. your starter battery probably has more usable energy than the lithium.
Lithium feels like a solution in search of a problem in this particular case :/

I was also concerned about charging over 1C, but the manufacturer said to hook it right up to the alternaotr and let the built in BMS do its thing. We'll see how that works out in the long run.

Also, the Ah rating on most modern Li batteries that I've seen is given in usable Ah instead of nominal - a blessing and a curse. This means my 40Ah battery is truly capable of 40Ah, but those of us looking into the changeover have a hard time getting out of the habit of automatically halving every Ah rating we see. Ultimately you're right and even 40Ah is not a massive increase given my time, energy and money invested - at this point I'm doing it for science.

and a refrigerator.

 

[dwh]

And weight savings?

 

[KevinK]

Yes, and weight savings. 14 lbs for usable 40Ah is pretty exciting.

On that note, our 20' of 2/0 cable arrived and I'm rechecking my maths to see if we really really need that much overhead and associated weight.

Tell me what you think:

15' from the house battery to the starter battery one way (including the ACR).
The starter motor (largest potential draw, if we have a self-jump start situation) is rated at 2kw, so at 12V would draw a max of 167amps (less amps if more volts are available).
I planned on making the positive run fused on both ends at 250 amp just to protect the run itself, and allow the 167 amps of the starter to flow without blowing a fuse (it's a DC motor, so there's negligible startup spike in amperage, but I figured an extra 90 couldn't hurt).

I looked at every chart and spreadsheet I could find and ended up choosing 2/0 for the ampacity.

What got me thinking I could use a lighter gauge wire is the chart on the 2/0 wire packaging from the manufacturer says 2awg has a 250 amp max at 25 feet and the 2/0 has a 450 amp max - way higher than we need.

Now I'm questioning all my maths, and wanting to downsize to 2awg.

Am I way off course?


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[dwh]

Not at all off base.

Wire ampacity tables are very different for different uses. For buildings, they usually assume "inside a conduit or raceway". But the same wire will have different (higher) ampacity ratings when used for "open air" or "chassis wiring".

Then there are temperature and time factors (as well as distance) to add to modify the ratings even further.

And welding cable has a its own rating system as well...

#2 could handle the starter load for a short time (few seconds) easily without melting.

 

[KevinK]

Thanks @dwh that's what I was thinking. And then I started looking at jumper cables and saw that most of the 16' jumper cables are actually 6 or 4 awg, with only a few 'heavy duty' ones at #2. I'm sure the open air rule applies, and jumper cables typically have super thick insulation.

I think I'm going to swap all this 2/0 stuff for #2.

I looked at voltage drop between the two sizes, and across 15 feet the delta in Vd is pretty small, even at high loads.

 

[KevinK]

Alright, well, everything is installed and functioning as designed. The problem is I didn't realize the design of the Blue Sea ML-ACR model 7622 that we spec'd is a dual sensing isolator. I thought it only sensed voltage on one side of the relay (in our case, the alternator side) so it would see when the starter battery was getting over 13 v and close the relay to start sending charge to the house battery.

That's not how it works.

It senses voltage on both sides, and when one side or the other exceeds 13v it closes the circuit so that whichever side is 'getting charged' can send its excess voltage to the battery that needs to be charged.

This is a problem because the nominal voltage of our Lithium battery is 13.2v - at rest it 'looks like' it's charging as far as the isolator is concerned. The ACR was not unlatching after I turned off the truck. So it kept trying to charge the lead acid starter battery with the 'excess' voltage from the LiPo.

I chatted with Blue Sea and told them I want to take apart the ACR and put in something in the sense circuit of the house side of the relay to trick it into thinking the 13.2v was 12.6 or so. He said that's a great idea except that any voltage regulator requires power flow to alter voltage. They used to make an isolator that was selectable between dual and single sense, but discontinued it when they released the ML series. It's a 9112 if you want to try and find an old one still for sale. It lacks the manual override switch of the ML series, so I'm gonna go ahead and open my ML to modify it.

Here's the plan - clip the lead that senses the house battery voltage and just connect it to the starter battery terminal so it only senses the starter and alternator voltages. We will lose the high and low voltage lockout function of the ML-ACR, but the lithium battery has its own high and low voltage protection built in to its BMS. The battery will protect itself in the case of an over or under voltage situation.

Let's see how this all turns out...


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[dwh]

D'oh!

Well...at least you'll still have a time delay after startup before tying, and the manual override.

Still, almost just as well to have gone with a dumb solenoid split-charge relay and saved a few bucks.

 

[Wyuna]

Really interested in how you go with this.

I've got a new 2017 VW Transporter with a factory Aux battery that i spec'd when i ordered it, and there is enough room under the seat to switch out the 75amp AGM and put in a 100amp Lithium for my campervan conversion.

Makes sense to have a battery, same size in a confined space, half the weight and can provide 80amps with a 80% discharge than 37.5 amps with a 50% AGM discharge.

The current charge is 14.55/14.56 volts into the AGM, so that is doable. although i'd have to rely on the integrated BMS to stop the charge once its full, not sure if the current factory relay is that smart to cut off.

Having said that, i haven't been able to measure if the Stop/Start brake regeneration affects the Volts while driving.

Its a bit hard to hold the multimeter on the batteries, drive around and also read the readings while driving.

I'll have to wire up a meter before the factory Aux battery at some stage to fully measure the impact of the Volts, under braking to see if they are stable or rise (the VW computer BUS controls this i believe, but do you think a VW tech could tell me a simple answer... no way)

"Edit" I should have pointed out that i will be also charging via solar, but as its a german made Votronic MPPT 350 Watt regulator with 4 different Lithium profiles, i believe thats under control and no chance of over charge from solar or from wrong charging.

 

[dwh]

[snip]...and no chance of over charge from solar or from wrong charging.

If the battery has a BMS then there's no chance anyway. The BMS is the battery's built-in charge controller.

 

[grizzlyj]

Hiya,

My limited research last year, looking at "Handybob's" website and following Gone with the Wynns, and being offered lithium batteries via one respected German builder but with charging from a different manufacturer that may or may not have suited, resulted in me thinking as and when I have a vehicle in front of me to bolt things to it will all be from the same manufacturer.

At that point last year for instance Sterling didn't have any battery to battery chargers with a "lithium profile", and Victron only had some suitable charger inverters in their range. If someone like Victron were suggesting only some of their products are ok I'm wary of stepping outside definite recommendations. But what do I know?!?

Hopefully we will be able to run a washing machine from them, and (or!), while the sun is shining, have aircon. If I have to, I can fire up a small genny and fully recharge from a washing day much more quickly with the higher current intake available. I wouldn't be considering either of these uses if we were to have AGMs.

One thing that will be incorporated is a lower charging current setting when hook up mains can't provide the full monty that lithiums could accept, quite common in some smaller campsites in Europe where one camper's hairdryer trips everyone around them. ie The supply only able to give everyone 5amps at 220v rather than 13amps max per socket you might expect.

Even on something with the space and carrying capacity, adding lightness will still help on every hill and soft spot

 

[Wyuna]

If the battery has a BMS then there's no chance anyway. The BMS is the battery's built-in charge controller.

I'm aware of that, i'm aware of what they do, and i don't believe anyone should install one without a BMS, i'd hate to see what a overcharged cell does in a campervan.

What i don't "know" is how long it switches it self off after reaching the BMS protection max volts, such as 14.6 (as an example).. is it minutes or hours, to protect the battery while underway. (haven't been able to find this out yet) hate to miss out on hours of charging between places, due to cut out (don't get me wrong i think its great that it is in the BMS), i might just have to run an aftermarket Lithium DC-DC charger, if the cut out periods are too large) i'd like to use what i have if it does work, if not plan B (Lithium DC-DC charger)

I have to do further testing of my VW T6 braking regeneration charge and to see if it does indeed affect the aux battery, (i'd like to think it only affects the Starter)


I have no issue with my Votronic solar controller, as like i said its advanced MPPT, with four individual lithium charging profiles as a lot of the lithium battery manufactures have different ways to charge their battery, and once it reaches 100% charge it's stop or float on a lower charge, depending on what lithium profile I've selected.

I'd like to think in a few years that factory aux battery systems with have a lithium profile, if they can come up with a lithium charging standard.

 

[Wyuna]

Hiya,

My limited research last year, looking at "Handybob's" website and following Gone with the Wynns, and being offered lithium batteries via one respected German builder but with charging from a different manufacturer that may or may not have suited, resulted in me thinking as and when I have a vehicle in front of me to bolt things to it will all be from the same manufacturer.

At that point last year for instance Sterling didn't have any battery to battery chargers with a "lithium profile", and Victron only had some suitable charger inverters in their range. If someone like Victron were suggesting only some of their products are ok I'm wary of stepping outside definite recommendations. But what do I know?!?

Hopefully we will be able to run a washing machine from them, and (or!), while the sun is shining, have aircon. If I have to, I can fire up a small genny and fully recharge from a washing day much more quickly with the higher current intake available. I wouldn't be considering either of these uses if we were to have AGMs.

One thing that will be incorporated is a lower charging current setting when hook up mains can't provide the full monty that lithiums could accept, quite common in some smaller campsites in Europe where one camper's hairdryer trips everyone around them. ie The supply only able to give everyone 5amps at 220v rather than 13amps max per socket you might expect.

Even on something with the space and carrying capacity, adding lightness will still help on every hill and soft spot

Did you check out the Votronic Range? German, but you can get them in the UK.

I got the Votronic Jupiter VPC panel (has the shunt for measuring everything), with all the matching switches, fuse boxes, solar regulator and tank sensors.

Its a tidy range and the support is great, sent me all the info in English when i asked for it.

The 2017 catalogue has all the new stuff, with the updated range which includes AGM and Lithium profiles (old range had Lithium only or Lead)

I'll be grabbing a Votronic lithium DC-DC charger if my factory Aux isn't suitable.

 

[dwh]

hate to miss out on hours of charging between places, due to cut out

From what I understand, with lithiums, once they reach the right voltage, that's it - they're full. They don't need absorb or float stages like lead-acid.

So if the BMS turns off the power flow to the cells, you're done. Doesn't matter how much longer you keep driving, because the cells are full.

(don't get me wrong i think its great that it is in the BMS), i might just have to run an aftermarket Lithium DC-DC charger, if the cut out periods are too large) i'd like to use what i have if it does work, if not plan B (Lithium DC-DC charger)

I have to do further testing of my VW T6 braking regeneration charge and to see if it does indeed affect the aux battery, (i'd like to think it only affects the Starter)


I have no issue with my Votronic solar controller, as like i said its advanced MPPT, with four individual lithium charging profiles as a lot of the lithium battery manufactures have different ways to charge their battery, and once it reaches 100% charge it's stop or float on a lower charge, depending on what lithium profile I've selected.

I'd like to think in a few years that factory aux battery systems with have a lithium profile, if they can come up with a lithium charging standard.

The BMS is a charge controller. Doesn't matter what the solar controller does, the BMS is going to determine the charge profile.

All the solar charge controller has to do is insure that the BMS is supplied with adequate voltage.

 

[DiploStrat]

Some Comments on Voltage Sensing, Intelligent Relays

A voltage sensing relay really has two different decisions to make: Close (combine) or Open (Separate). Depends on where you start.

With two lead acid batteries with a resting voltage, the formula is pretty well known. You combine when either battery is over X voltage, typically 13.2v. As most lead acid batteries rest at under 13v, this pretty much requires that there be a real charge source connected. (We will ignore time delays, etc.) So start your engine in the morning and the voltage of your starter battery will drop as it cranks and then rebound as the alternator starts charging. Turn off the engine and the batteries will NOT separate, they are not supposed to.They will not separate until the resting voltage drops to the separate voltage, typically 12.7v. Why 12.7v? Because that is the nominal full charge voltage of a 12v battery. (Some Australian relays drop this to 12.5v, to allow a reserve for winching.) How fast will your battery voltage drop? Depends on your loads. Imagine you have refrigerator running and it will drop faster. Turn off all of your loads and it will take many hours. Add in solar, attached to camper battery and the relay may not open for weeks. And that is the issue, once the relay is closed it senses from both sides. Why? Because, as dwh never tires of reminding us, it is a circuit. So "dual sensing" is only an issue when the relay is open, not when it is closed.

So now we have the challenge of a lithium battery of some sort. These are usually assembled of cells that add up to a resting voltage of about 13.2-13.4v. As they say in West Africa, "What to do?"

In my research, NOT supported by personal tests, the answer varies.

-- Many manufacturers tell you to ignore the problem. Trick is that I suspect that most assume a key controlled relay which tends to make the issue moot.

-- Others, like Victron, sell an intelligent relay with higher combine and separate voltages. You can do the same thing yourself by using a Magnum Smart Battery combiner. This beast will pass 25A or switch a relay. It lets you adjust the combine/separate/maximum voltages. I have been using one for years. In my case, the only draw back is that the maximum voltage is 15v, so my batteries may separate when the weather is really cold and the voltage regulator calls for voltages over 15v. I have a manual override switch.

-- Solar may also make the issue moot, that is, with a solar charger connected, your de facto float voltage is going to be 13.6v or higher, so the relay will be closed most of the time. (This is my experience.) A fully charged starter battery will be only a tiny drain; so small that you can ignore it.

There are BMS and there are BMS. Indeed, this may be the real difference between different lithium batteries. Most that I am familiar with only have brick wall cut offs at the top and bottom, that is high or low voltages. At least one, however, sells products to get between the alternator and the regulator to adjust as needed.

The BIG issue, and one where I get conflicting answers, is whether a lithium battery connected to a lead acid battery will tend to equalize the way lead acid batteries do. Some lithium vendors tell me that they will not. Indeed, they say, that is the big reason for the cell balancing circuitry in a lithium cell - the cells will not auto balance the way lead acid batteries do. Here, again, I have NO real world data.

All of the above offered for your consideration. I, too, would love to hear from those who are actually running different setups.