Hybrid Lithium AGM setup

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werdna asked May 16, '22

Hybrid Lithium AGM setup

I am planning a battery upgrade for my new 4x4 which comes factory with a single 105ah AGM battery.

Due to space constraints I am planning to add around 100ah of LIFEPO4 prismatic cells to beef up the capacity.

All the loads i want to run are already wired off the AGM battery and I don't want to move all the circuits over to the lithium battery. The vehicle has a very complicated electrical system that I would like to leave as standard as possible.

So the plan is to have the lithium charging through a DC/DC while the engine is running, then when the engine is off the DC/DC will disconnect and the lithium will be used to charge the AGM as my camping loads slowly deplete it. In theory the AGM will stay at 100% SOC until the lithium is discharged. Then the lithium cuts out and I am discharging from the AGM.

I think this will work but welcome any comments, I'm a little out of my depth here.

The current plan is to use another, smaller DC/DC charger from the lithium into the AGM. Although looking at the voltage curves I wonder if I could save some money by switching to a cheaper DC/DC converter set at 12.8v, or even just a Victron Smart Battery Protect. With the battery protect only, the AGM should see a float charge of 13.8v from the full lithium, dropping down to 12.8v as it discharges. Then, I guess, they both both discharge together until the BMS cuts the lithium out at 12.0v. By which time the AGM is empty anyway and I need to run the engine or stop drawing power.

Is this reasonable? Or should I stick with a second DC/DC charger?

Lithium Battery AGM Battery

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Answer 1 · 2022-05-16T09:59:10Z

klim8skeptic answered · May 16, 2022 at 09:59 AM

Your truck has a single 105ah agm battery for starting and auxiliary stuff?

Inconceivable!

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werdna commented · May 16, 2022 at 10:51 AM

It's not a truck. Just a small 4x4. Not much capacity for auxiliary stuff, but it can run my camping needs for a day/night and start no problem the next day from ~12.2v

I would like to add more capacity so I can go longer without charging. If I can keep the AGM near full for extra starting reserve that'd be great.

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klim8skeptic ♦ werdna commented · May 16, 2022 at 11:45 AM

I would like to add more capacity so I can go longer without charging. If I can keep the AGM near full for extra starting reserve that'd be great.

Add a small b2b charger and a mppt to the aux battery.

Should your start battery fall over, jumper leads are cheaper than a second b2b charger.

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werdna klim8skeptic ♦ commented · May 17, 2022 at 11:20 AM

All my loads are already wired off the main battery and I can't easily separate them. I want to keep drawing current from the main/AGM battery and just have it be constantly topped up by the aux/Lithium.

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Answer 2 · 2022-05-16T12:42:16Z

pwfarnell answered · May 16, 2022 at 12:42 PM

Recently discussed with lots of detail, especially if you follow the link provided by wildebus. See

https://community.victronenergy.com/questions/119423/mix-agm-and-lifep04.html

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werdna commented · May 17, 2022 at 11:40 AM

Thanks. I actually read that thread and all the info on @Wildebus site right after posting this one. I also read the info from Emily and Clark(?) some time ago when I was first kicking the idea around.

Seems like these guys are having good results combining the 2 chemistries.

These guys are getting by with less protective "gizmos" than I had planned which is reassuring and may help to keep my costs down.

I think I should still run a DC/DC charger into the lithium to protect my alternator in case the SOC is low and it pulls 100 amps or more. My alternator is 190 amp and water cooled so could probably handle it, but is a huge job to replace if it fails so I would rather err on the side of caution.

So I can't just connect the batteries in parallel, and i guess the DC/DC chargers block current in both directions when turned off so I need something to manage current flow from the Li to the AGM. My first thought was to use the victron smart battery protect, set to cut the Li out at say 10% SOC. This is a budget friendly option and based on wildebus experiments should function fine.

But I would still like somebody to hold my hand and tell me it will work before going ahead :)

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Answer 3 · 2022-05-23T07:38:47Z

werdna answered · May 23, 2022 at 07:38 AM

Well I ordered all the parts today so will know soon if works as intended. I went with the Smart Battery Protect instead of the second DC/DC charger, saved about $200

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Answer 4 · 2022-06-08T21:43:54Z

werdna answered · Jun 08, 2022 at 09:43 PM

OK battery pack is assembled and ready to put in the car :D

My last question is where to input the solar? I currently have a "dumb" MPPT regulator which is fixed on an AGM charge profile (often sits on 14.4v output in full sun). Panel only comes out when we are stopped and engine off.

If I wire it to the lithium and it's at 100% SOC, I'm worried the 14.4v will overcharge and damage the battery. Although I guess the BMS will disconnect to stop this?

If I wire it to the AGM (which is more convenient), I could have 14.4v on the "out" side of the Victron Smart Battery Protect and 13.8v on the "in" side. Will this cause current to flow backwards through the SBP and damage it?

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Answer 5 · 2022-11-02T19:58:55Z

Well I've had the hybrid battery setup running for a while now and it's working nicely.

Turns out the Victron Smart Battery Protect is a bit of a POS and doesn't actually fully isolate the battery in "off" mode. There was always a few volts still registering at the output and upsetting my careful plan.

I also decided that I liked the idea of a DC-DC charger from the lithium back to the AGM as it would maintain 100% SOC on the AGM all the way up until the Li was empty. Basically the main battery would always be on trickle charge while the car isn't being driven. Would be ideal to power my camping accessories and also maintain the AGM battery in between trips.
Didn't have room for another charger but I noticed that the settings in the Orion were not too dissimilar for AGM and LiFePO4 chemistries. Seemed redundant to have 2 identical chargers with near identical settings that never ran at the same time so I set about using the one Orion to charge either battery depending whether the engine is running or not.

I used 3 x 60 amp changeover relays and an Arduino to achieve this.
When the engine is off (no oil pressure), relay 1 NC connects the charger input to the LiPO and relay 2 NC connects the charger output to the AGM. Relay 3 NC connects the charger ground.
When the oil pressure switch goes high, the Arduino first drops out the remote input to the charger, disabling charge. There's a 4 second delay until it fires relay 3, dropping the ground from the charger and fully de-powering it. Another 4 second delay and it fires relay 1 & 2, connecting the charger input to the AGM/Alternator and the output to the LiPO. Another 1 second delay and it drops out relay 3 to reconnect the ground. Another 1 second and it connects the charger remote to begin charging.
When the engine turns off again the same thing happens but we end up in a low power state with no relay coils energised.

Here are my charge settings in the Orion. Its main job is to fully charge the LiPO, and maintain a full state in the AGM. It should never really have to charge a flat AGM so the bias is more towards charging the LiPO.
Absorption voltage: 14.2v
Float voltage: 13.5v
Bulk time limit: 10 hours
Re-bulk voltage offset: 0.1v
Adaptive absorption time
Maximum absorption time: 2 hours
Input voltage lock-out 12.4v-12.8v
I'm open to any input on tweaking these settings for a best compromise between the 2 chemistries.

And here are some pics of the battery build and and install under the driver's seat. Things are packed in tight! The space used was freed up by removing the obsolete 2000's era factory navigation DVD setup and telephone module. Not an inch of wasted space in these cars!

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