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Leisure Battery
- Thread starter Ian M
- Start date
Kevin John
Member
Another item I left off… The ignition wire that turned on the K120 is not presently connected to my Orion XS. I plan to connect it next time I’m in the battery compartment so the Orion will turn off immediately when I cut the engine. It currently keeps pulling amps from the start battery until the voltage drops to a preset level. The Orion then turns off. When I connect the ignition wire, it will shut off when I cut the engine.
Hi everyone,
I have a 2018 Marco Polo and recently replaced the original leisure battery with an Ultimatron 180Ah LiFePO₄ battery.
Although the battery is advertised as a direct replacement and has virtually the same dimensions as the original AGM battery (approximately 355 × 175 × 188 mm), I found that the base flange was slightly too wide to slide into the battery guide rails. I had to carefully remove about 1 mm from each side of the flange only using a plastic file and sandpaper. Do not file the battery case itself—only the flange, and do it carefully to avoid damaging the battery enclosure or internal components.
So far, I haven't had to make any other modifications. The battery powers all original systems without any issues, including the Westfalia control unit, which reports virtually the same battery voltage as the BMS. I'm currently monitoring the charging/discharging behavior as well as standby performance.
Although Ultimatron states that the battery is a direct replacement for AGM batteries and that the existing charging system does not need to be modified, I was naturally curious to see how the charging process would behave in practice. (Source)
To test this, I started with the battery already at 96% SOC. When I started the engine, the initial charging current peaked at approximately 100 A for about a minute, then dropped to around 50 A, and gradually decreased to approximately 12 A. Once the battery reached 99–100%, the charging current dropped to 0 A (or close to it), exactly as expected.
I was also monitoring the battery temperature during charging. So far, the BMS has not reported any abnormal temperatures. The battery temperature appears to be influenced more by the ambient temperature and the heat from the engine than by the charging current itself.
My next test will be to discharge the battery to a much lower state of charge and observe the complete charging curve, including the average charging current over a longer period.
So far, everything looks very promising. I'll post updates if I discover anything new or if any issues arise.
I have a 2018 Marco Polo and recently replaced the original leisure battery with an Ultimatron 180Ah LiFePO₄ battery.
Although the battery is advertised as a direct replacement and has virtually the same dimensions as the original AGM battery (approximately 355 × 175 × 188 mm), I found that the base flange was slightly too wide to slide into the battery guide rails. I had to carefully remove about 1 mm from each side of the flange only using a plastic file and sandpaper. Do not file the battery case itself—only the flange, and do it carefully to avoid damaging the battery enclosure or internal components.
So far, I haven't had to make any other modifications. The battery powers all original systems without any issues, including the Westfalia control unit, which reports virtually the same battery voltage as the BMS. I'm currently monitoring the charging/discharging behavior as well as standby performance.
Although Ultimatron states that the battery is a direct replacement for AGM batteries and that the existing charging system does not need to be modified, I was naturally curious to see how the charging process would behave in practice. (Source)
To test this, I started with the battery already at 96% SOC. When I started the engine, the initial charging current peaked at approximately 100 A for about a minute, then dropped to around 50 A, and gradually decreased to approximately 12 A. Once the battery reached 99–100%, the charging current dropped to 0 A (or close to it), exactly as expected.
I was also monitoring the battery temperature during charging. So far, the BMS has not reported any abnormal temperatures. The battery temperature appears to be influenced more by the ambient temperature and the heat from the engine than by the charging current itself.
My next test will be to discharge the battery to a much lower state of charge and observe the complete charging curve, including the average charging current over a longer period.
So far, everything looks very promising. I'll post updates if I discover anything new or if any issues arise.
Last edited:
Van Diesel
Active Member
great ondra first hand expirience relayed,, D keep posting how it goes pls .. ! D so u didnjt get the new charger controller at all.. just let the bms handle it assume,,, D
this surprised me ..... my agm charges almost nothing after 80-90 few amps,,,
quote:
To test this, I started with the battery already at 96% SOC. When I started the engine, the initial charging current peaked at approximately 100 A for about a minute, then dropped to around 50 A, and gradually decreased to approximately 12 A. Once the battery reached 99–100%, the charging current dropped to 0 A (or close to it), exactly as expected.
this surprised me ..... my agm charges almost nothing after 80-90 few amps,,,
quote:
To test this, I started with the battery already at 96% SOC. When I started the engine, the initial charging current peaked at approximately 100 A for about a minute, then dropped to around 50 A, and gradually decreased to approximately 12 A. Once the battery reached 99–100%, the charging current dropped to 0 A (or close to it), exactly as expected.
Last edited:
Van Diesel
Active Member
the controller is more for the lack of charge after 80 .... read my edit ! D solid info m8. D
Van Diesel
Active Member
app the ai is not happy about it still.. D
If you see someone in the Marco Polo community drop an Ultimatron battery straight in with no DC-DC charger, and it "works" perfectly for them, they are not imagining it. It genuinely can function in the short term, but it is working because of temporary physical loopholes, not because the BMS is tricking the van. [1, 2]
A straight drop-in replacement operates without a booster for a few specific technical reasons:
1. The Headlights / Windscreen Heater Loophole
Smart alternators determine their voltage output based on the electrical load of the vehicle. If "that guy" drives with his headlights turned completely on, or runs the climate control on high, the Mercedes computer panics about the high load. It commands the alternator: "Ignore fuel-saving mode, push a constant 14.2V–14.4V to keep the vehicle electronics alive." [1]
2. High Current "Regenerative" Bursts
Mercedes Euro 6 systems use a mild regenerative engine-braking strategy. When coasting down hills or braking, the alternator deliberately spikes up to 14.8V+ to harvest energy. Because an Ultimatron has zero internal resistance, it will greedily swallow 100+ Amps in those short bursts. This makes it look like the van is charging the lithium battery incredibly fast, even though it only happens during deceleration phases. [1, 2]
3. The Thick Factory Wiring Restrictor
You might wonder why a 100 Amp draw doesn't instantly blow the vehicle's factory fuses or melt the wires. The factory wiring traveling from under the driver's seat to the leisure battery area creates natural voltage drop. [1, 2, 3]
Why It "Works" vs. Why It Fails Long-Term
Many Marco Polo owners run a drop-in lithium battery with a simple relay setup for a few trips without an issue. However, most eventually notice the battery cell tracking in the app getting wildly unbalanced, or find the battery stalling at 80% capacity on long highway drives, leading them to ultimately add a Victron Orion XS later on. [1, 2]
If you plan to test a straight drop-in setup first, I can tell you what to look for in the Ultimatron Bluetooth app to see if your cells are becoming unbalanced. Would you like to know how to check for that?
If you see someone in the Marco Polo community drop an Ultimatron battery straight in with no DC-DC charger, and it "works" perfectly for them, they are not imagining it. It genuinely can function in the short term, but it is working because of temporary physical loopholes, not because the BMS is tricking the van. [1, 2]
A straight drop-in replacement operates without a booster for a few specific technical reasons:
1. The Headlights / Windscreen Heater Loophole
Smart alternators determine their voltage output based on the electrical load of the vehicle. If "that guy" drives with his headlights turned completely on, or runs the climate control on high, the Mercedes computer panics about the high load. It commands the alternator: "Ignore fuel-saving mode, push a constant 14.2V–14.4V to keep the vehicle electronics alive." [1]
- The Result: The lithium battery catches a free ride on that high voltage signal and charges fully, mimicking a DC-DC charger without actually having one. [1]
2. High Current "Regenerative" Bursts
Mercedes Euro 6 systems use a mild regenerative engine-braking strategy. When coasting down hills or braking, the alternator deliberately spikes up to 14.8V+ to harvest energy. Because an Ultimatron has zero internal resistance, it will greedily swallow 100+ Amps in those short bursts. This makes it look like the van is charging the lithium battery incredibly fast, even though it only happens during deceleration phases. [1, 2]
3. The Thick Factory Wiring Restrictor
You might wonder why a 100 Amp draw doesn't instantly blow the vehicle's factory fuses or melt the wires. The factory wiring traveling from under the driver's seat to the leisure battery area creates natural voltage drop. [1, 2, 3]
- That resistance acting on the thin wire naturally chokes the current back, preventing the battery from pulling the full, dangerous amperage the alternator is capable of producing. It acts as a crude, accidental bottleneck that protects the factory system. [1]
Why It "Works" vs. Why It Fails Long-Term
| What the User Sees (Short-Term) | What is Actually Happening (Long-Term) |
|---|---|
| The battery charges to 85% or 90% fast. | The cells never reach a true 14.4V top-charge, meaning the BMS cannot balance the internal cells. Over 6–12 months, the cells drift apart, reducing overall capacity. |
| The van drives normally without blowing fuses. | On long, hot summer drives, the alternator runs at maximum thermal capacity trying to supply both the van and the hungry lithium battery, accelerating alternator wear. |
| The Ultimatron Bluetooth app shows it's working. | On long highway cruises where the engine is flat, the alternator drops to 12.6V. The lithium battery's natural 13.3V means it slowly discharges back into the van or sits stagnant. |
If you plan to test a straight drop-in setup first, I can tell you what to look for in the Ultimatron Bluetooth app to see if your cells are becoming unbalanced. Would you like to know how to check for that?
Van Diesel
Active Member
m not super into your issue,,, but this can replace the mercedes one i think,,,, so u always get a good charge on the lipo. perhaps wiring this in, in the correct order would solve your problem?
its when driving yea but dosent all charge current go in the relay? the thing that decides which battery to charge when...or gets signalled about it.... not sure how it all works,, ! D
Charge booster / charge controller for Mercedes Marco Polo, Horizon, Activity W447, perfect charging while driving for your camper - incl. installation The Mercedes Marco Polo has a regulated alternator (Euro 5 and more), which ensures that the battery is charged poorly or not at all
www.1sternehotel.de
better put one of these in b4 the cables catch a fire m8,,,, they are doubled insulated and no so much cobber as one would think,,,
its when driving yea but dosent all charge current go in the relay? the thing that decides which battery to charge when...or gets signalled about it.... not sure how it all works,, ! D
Votronic charge booster / charge controller for Mercedes-Benz Marco Polo, Horizon, Activity W447, perfect charging while driving for your campervan
Charge booster / charge controller for Mercedes Marco Polo, Horizon, Activity W447, perfect charging while driving for your camper - incl. installation The Mercedes Marco Polo has a regulated alternator (Euro 5 and more), which ensures that the battery is charged poorly or not at all
better put one of these in b4 the cables catch a fire m8,,,, they are doubled insulated and no so much cobber as one would think,,,
Van Diesel
Active Member
or check that the cables are 50 sq inch.. they allready take a load of 80 amps so might be
Van Diesel
Active Member
also there might issues wtih the shore power chrger ,, its as ususal very confusing .... imma gonna let it rest again lol and just start the dang engine
Well, there are multiple weird assumptions. I also watch balancing issues to check if balancing goes well.
1 . I cannot confirm the first point, I am at the moment at 10–20 mV diff, but I am at the starting point. This may happen under some conditions. It depends on cell quality and BMS design. It can be true or not for different products.
2. that's possible even with AGMs. As I pointed, charging current is decreasing with time and is steady after short time from my observation - from BMS app.
3. is quite brave assumption by AI - going to test it. But of course, when the smart alternator reduces its output voltage below the battery's charging threshold, the LiFePO₄ battery may stop charging. If the onboard leisure loads continue to consume power, the battery's state of charge can slowly decrease during the drive - possibly - going to test it.
All of it is an assumption. That's why I am curious. Only concern I have is a temperature of the system right now, that's why I go step by step.
The Rocket
Member
Details are on the battery itself. Very easy to replace I am sure there is a video on You Tune. Mercedes price is very reasonable.
Van Diesel
Active Member
well he claims that place like stjerne hotel ,,, in germany reprogram the car via the thignie ,,,,, the problem is no resistance within the battery the carrge controller is a small expence,,,, which just migh leave shorecharging the last issue ,,,, bur rite its confusing as f.... just be observant at least ,,, switching the shore charger to lithium might yield problems for the main battery etc etc more and more problems lol..... i dunno .. imma sure u will watch it like a hog ,,,,so ... let us hear how it goes down the road or talk to the ai urself .... D
hes not always rite .... though but when it comes to safety i dont overrule him just reask him u absolutly sure about this or that ? naa im not he says and start re investigating ,,, lmao
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