Float charge AGM from Lithium

[Ann3x]

Sorting out some electrics in my van and I'm considering adding one of the votronics trickle chargers between my Lithium Leisure Battery & the AGM starter.

My van is not a daily driver and the starter has gone flat a few times (it's not a Moll... just gaps between use I guess). There is nothing critical running off the Leisure battery d2d so my idea was to trickle charge the AGM from the Lithium to maintain the starter battery for longer. When i take the van out the Lithium will then recharge via the B2b & hay presto - I've got an extended lifetime on my starter.

The main concern I have is that the float voltage of the Li might be too different to the AGM so the lithium discharges constantly with no actual benefit. Apparently the Votronic needs a net different of 0.7V between the batteries to activate. Anyone know if this is a relevant concern or any other reason NOT to do this?

Im aware that a better long term solution is dual output solar - just dont want to do that quite yet.

 

[Dellmassive]

You cant use these for Lithium !!!!

(due to the higher static/rest voltage of Lithiums)

Lithium Leisure Battery & the AGM starter.

VOTRONIC - StandBy Charger

Innovative Elektronik für den mobilen Einsatz

www.votronic.de

https://www.votronic.de/images/Manuals/10.%20Peripheral%20Units/3065_6065_e.pdf

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[Dellmassive]

maybe look at one of the dash top mobile solar solutions . . .

[Guide] Mobile Solar Panels ? . . . - How I Did It -

Mobile Solar Panels ? . . . - How I Did It - There is loads of background to this . . . . . but for now here is a vid for you all. =] Just remember W = V*A and as a general rule a good 100W panel will give you up to 6A charge with good sun. 25w = 1.5A 50W = 3A 100w = 6A 200w = 12A 300w = 18A...

www.t6forum.com

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[Ann3x]

Yeah i thought the same but not what im hearing from the manufacturer:

This b2b charger will not work for charging your lithium battery from your starter battery, but it will be able to charge your starter battery from your lithium leisure battery. This charging will start when the voltage difference between the two batteries is 0.7V. ​

​

The reason that warning is put in the instructions is because it is a lead acid battery charger and cannot charge up a lithium battery. But as the current is low and it is the lead acid starter battery you intend to charge up, this shouldn't be an issue. ​

​

Just to clarify, this b2b charger is suitable for your system and won't damage either battery as long as you are using it to charge the lead acid starter battery from your lithium leisure battery.​

I know the manual says it wont work but that looks to be mainly a catch all to stop people using it as a cheap b2b (AGM>Li) - which it clearly isnt. So WHY wouldnt it work is the question?

 

[Dellmassive]

what Li & B2B have you got?

i cant see you in the list . . . .


add your self in . . . - What Solar Panel/controller Dc-dc Leisure Battery - Have You Got ?

remember to add you setup, inverter now added to list =]


***************************************
Copy & Paste the below to add your setup:

Solar Panel:
Mobile Panel:
Solar charge controller:
DC-DC charger:
EHU Charger:
battery bank:
Battery Monitor:
Inverter:
other:

***************************************

 

[Ann3x]

what Li & B2B have you got?

i cant see you in the list . . . .


add your self in . . . - What Solar Panel/controller Dc-dc Leisure Battery - Have You Got ?

remember to add you setup, inverter now added to list =]


***************************************
Copy & Paste the below to add your setup:

Solar Panel:
Mobile Panel:
Solar charge controller:
DC-DC charger:
EHU Charger:
battery bank:
Battery Monitor:
Inverter:
other:

***************************************

Solar Panel: -
Mobile Panel: -
Solar charge controller: -
DC-DC charger: Sterling BB1230
EHU Charger: Optimate Lithium 5A
battery bank: SuperB 90Ah Lithium
Battery Monitor: - (Super B has bluetooth module built into BMS)
Inverter: -
other:

Ill post it in that thread too.

Im thinking about switching out the Sterling & going full Victron (or maybe something else) so when I eventually add in solar i can manage it better from one app. Interested in your views on battery monitoring actually.... (asked in the monitoring thread).

 

[Dellmassive]

right . . . ok.

well technically all that in is the box is a diode and a resistor (thats why it only has 2wires and no ground reference.

the idea is that that if the source side is higher than the load side . . . . current will flow limited bu the internal resistor.

There will be a 0.7v drop across the internal diode and some heating effect....

so in the setup of Lifepo4->AGM the Li will always have a higher voltage that the AGM.

My Valence LI rest at around 13.9v . . . . to in effect there will be current flow to the AGM until it reaches is max absorption . . . mostly likely 12.6v ish.


im practice i would assume you you constantly see the Li voltage (minus 0.7v diode volt drop) across the starter battery/van electrics.


will this cause your B2B to start cycling? or starting up?


this has be raised before and one sugestion was to install a isolator switch inline so it can be dissabled?


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Solar System 2 Battery System Help

Yeh . . . . . you can pickup a main starter battery feed from the rear of the center console fuse box, but most people prefer to go direct to the starter battery(prob best option). But basically, under the cab floor mat to the center cable tunnel, up behind the glove-box, via grommet that...

www.t6forum.com

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[Ann3x]

right . . . ok.

well technically all that in is the box is a diode and a resistor (thats why it only has 2wires and no ground reference.

the idea is that that if the source side is higher than the load side . . . . current will flow limited bu the internal resistor.

There will be a 0.7v drop across the internal diode and some heating effect....

so in the setup of Lifepo4->AGM the Li will always have a higher voltage that the AGM.

My Valence LI rest at around 13.9v . . . . to in effect there will be current flow to the AGM until it reaches is max absorption . . . mostly likely 12.6v ish.


im practice i would assume you you constantly see the Li voltage (minus 0.7v diode volt drop) across the starter battery/van electrics.


will this cause your B2B to start cycling? or starting up?


this has be raised before and one sugestion was to install a isolator switch inline so it can be dissabled?


************

Solar System 2 Battery System Help

Yeh . . . . . you can pickup a main starter battery feed from the rear of the center console fuse box, but most people prefer to go direct to the starter battery(prob best option). But basically, under the cab floor mat to the center cable tunnel, up behind the glove-box, via grommet that...

www.t6forum.com

***********

The Sterling has an ignition live in (which i think stops it running when van is ign off) so my assumption was that this function would prevent the B2B cycling up whilst parked up to redress the effect of the trickle charge... It would only do this when im running the van - which is fairly safe if so as total charge is going up via the alternator which would pretty much negate the efficiency loss of the loop.

 

[Dellmassive]

this is worth a look too:


Ctek250se Solar Input Charging Both Batteries Via Victron Mppt
Here's a question. I have a CTEK250SE installed and also use a Victron 75/15 MPPT controller with a 160w solar panel. During the current travel restrictions, I see my starter battery dropping sometimes down to 11.5v so leave it running for 15mins to get it back up into 12v territory every 3-4...

www.t6forum.com
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Dellmas




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another mention of joining batteries here:


Hillside birchover split charge relay
Anyone know what type of split charge relay is fitted to birchover. Does it work in reverse ie when leisure battery full will power from solar panel or hook up be charging starter battery. As not using van now just worried about flat starter battery.

www.t6forum.com

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[Dellmassive]

The Sterling has an ignition live in (which i think stops it running when van is ign off) so my assumption was that this function would prevent the B2B cycling up whilst parked up to redress the effect of the trickle charge... It would only do this when im running the van - which is fairly safe if so as total charge is going up via the alternator which would pretty much negate the efficiency loss of the loop.

yep total agree with the logic.

BUT

some B2B Will start charging with a higher starter voltage REGARDLESS of the D+/IGN/BCM Run feed.

infact @Lowlec is using this as a feature to keep both his batteries charged - Ctek250se Solar Input Charging Both Batteries Via Victron Mppt

his solar MPPT is charging the starter battery at rest, which raises the voltage high enough to make the DC-DC kick-in while static!


so i think you need to get it and try . . . . the Sterling may/may not auto start?

you could fit a isolator to disable the unit, when not parked up. etc etc

edit: or you could run it through an ignition controlled relay so its only connected when the van is shut down?


Oh- nice Lifepo4 battery BTW =]

 

[Dellmassive]

you will need to test the idea and monitor the heat caused by it running 24/7 - which would be a factor of the SOC SOH and voltage of the starter battery . . . .


ie when parked up and opening the door will make the modules wake up and interior lights come on, which will draw a load, which will cause current flow via the votronic due to the higher voltage of the Li.

 

[Ann3x]

you will need to test the idea and monitor the heat caused by it running 24/7 - which would be a factor of the SOC SOH and voltage of the starter battery . . . .
ie when parked up and opening the door will make the modules wake up and interior lights come on, which will draw a load, which will cause current flow via the votronic due to the higher voltage of the Li.

I reckon ill give it a try. Sounds like the worse case scenario is that both batteries will drain (due to efficiency loss with the B2b), no actual damage seems likely.

If it doesnt work as long as I use decent gauge wire, I can always just reuse the wiring for a dual output solar controller down the line.

 

[Ann3x]

Reading the Sterling manual it does indeed look like the B2B can activate ANYWAY even with IGN live off. It looks like this behaviour will happen until the starter goes under 13V.

So this means that wiring this unit as I planned could result in the following behaviour:

1. Starter discharges to a point where Starter / Leisure diff > 0.7V
2. Leisure restores charge via trickle until starter hits 13.2V
3. When starter voltage > 13.2V B2B will start to charge the Leisure Battery
4. When starter voltage < 13V B2B will turn off

The above cycle would then repeat until the Leisure battery is exhausted and the onboard BMS kicks in.

The kicker here is that afaik an at rest AGM output maxes at around 12.9V so the B2B's voltage threshold will never be reached without the alternator (or another charger eg @Lowlec MPPT) connected. If that is true then the sequence above becomes much simpler - the trickle will just work until the Lithium is discharged to a point where the voltage diff of 0.7V is not present. At which point both batteries would be basically flat anyway.

Logic makes sense or am i missing something?