bjornm avatar image
bjornm asked

Why don't the MPPT chargers have battery voltage sense leads?

I'm looking to buy one of the SmartSolar Charge Controllers, probably the 250/70.

Then I notice that it has no connection for battery voltage sense leads, and that you have to buy a separate dongle that sends this via Bluetooth. I'm very confused about this design decision, since a battery sense is essential to make any controller work in a reliable way. Why make it a separate product, and why send it over Bluetooth, when it is just two simple cables? Can someone explain?

MPPT Controllersvoltagesensor
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4 Answers
Alexandra avatar image
Alexandra answered ·


For most installs they are unnecessary.

If you have a really long run to the battery or undersized cables and get voltage drop, then the additional hardware is needed.

I can tell you most of the installs I have done don't need to use voltage sense extras and they are quite fine and have been for years.

Maybe the Smart battery Sense makes sense for cold cut off? Again, not all installs need it.

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bjornm avatar image bjornm commented ·

Thanks for the reply. Let's say the charger is mounted 2 m from the battery bank via 35 mm2 cables and 70 A runs in those cables. That's 2 milliohms of resistance causing a 0.14 V drop in the voltage that the charger is unaware of. So I won't be charging to 27.60 V, I will be charging to 27.46 V. I'm just surprised, because every charger I have used so far has battery voltage sense leads, and it is such a simple thing to offer. Bluetooth seems very unreliable, so the only real solution is to buy larger cables.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ bjornm commented ·
You only have that (very small) voltage drop if it is charging with full power.

After reaching the absorption voltage the current will go down and so the voltage drop and you batteries will reach 27,6V at the end.

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bjornm avatar image bjornm Matthias Lange - DE ♦ commented ·

Yea that might work fine in your case. My charge profile for LFP (Lithium) doesn't have an absorption stage. I bulk charge to a specific voltage, then go straight to float. And I would like the charger to get to that specific voltage every time, no matter how much power my solar array gives. I agree that this is unimportant details to many people. I might get this charger anyway. But I'm just surprised that a famous brand like Victron doesn't have this feature. And especially surprised that it is solved through a Bluetooth connection.

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klim8skeptic avatar image klim8skeptic ♦ bjornm commented ·

@BjornM And especially surprised that it is solved through a Bluetooth connection.

Another option if you were to use a GX device for monitoring & control, would be to use DVCC to transmit voltage, temperature and current data between devices.

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bjornm avatar image bjornm klim8skeptic ♦ commented ·
Thanks, will look into it.
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snoobler avatar image snoobler bjornm commented ·

Victron is in good company.

Outback and Midnite don't have this feature, and there is no bluetooth solution.

So it sounds like Victron is a step ahead in that regard - multiple options to pass voltage, current and temperature.

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bjornm avatar image bjornm snoobler commented ·
Outback uses voltage sense cables. Morningstar too. Midnite I don't know about.
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snoobler avatar image snoobler bjornm commented ·
Outback FM-60/80 don't, and that's probably their most prolific controllers. Spendy too.

Morningstar is great, but you'll pay a 2X premium for 60% of the performance of a Victron. I'll take the $50 SBS option and save hundreds.

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furyjk98 avatar image furyjk98 bjornm commented ·

I would just offset the small voltage that is needed at the terminals of the battery in MPPT settings to reach the desired voltage.

However, if you are an advanced user, and installed a fully functional Victron setup with inverters, multiple batteries connected via a bus bar, with multiple MPPTs. Buying a battery sense will make sense with the setup since all those components can access the data via VE.Smart networking.

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bjornm avatar image bjornm furyjk98 commented ·
You can't offset the voltage because there is no way to know what the load from the inverter is. Lets say the charger gives 70 A and the inverter 0 A. Then you have one specific offset. If the inverter is at 70 A, then you have a very different offset.
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kevgermany avatar image kevgermany ♦♦ bjornm commented ·
Assuming you added a voltage offset, the draw of the inverter will prevent the battery from charging. Once the inverter load decreases, the batteries will draw more current. And will reach your desired voltage.

But really this is a pointless discussion. Except for occasional situations there's no need or point for separate cables. And it's not worthwhile to increase the complexity, redesign the product and increase its price as a result.

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regman54 avatar image
regman54 answered ·

@BjornM To address your 'why' question I'll offer my perspective.

I have a modest Victron solar setup for my travel trailer. Bluetooth allows my various Victron components to communicate with one another wirelessly for a cleaner, easier install. When we are not using the trailer it is parked year round in our driveway. The BT battery sense allows me to monitor the battery bank's rested SOC without stepping outside. It's the only reason why I have one. For some installs the sensor's battery temperature feature can also provide important information to other pieces of Victron hardware. In short it's not just a voltage sensor.

As already mentioned not everyone needs a battery sense as the CC can, in most cases, operate just fine without one. A built-in battery sense within every CC is not a feature I would want to pay extra for if I didn't need it. But I am however glad to have one available to me should I want it. In fact it's the Victron hardware's Bluetooth abilities that prompted me to move over to the brand.


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bjornm avatar image bjornm commented ·
I agree that it is a plus for some customers. Just not for me. I prefer old fashion cables that always work. Only very cheap chargers don't have battery sense cables. And Victron chargers.
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Alex Pescaru avatar image
Alex Pescaru answered ·

All batteries, no matter the technology, reach a point towards the end of the charging process where the current draw is minimal.

Of course this depends on the final, float, voltage setting, but no matter, if the float voltage is properly set, then the current draw is very small at the end.

So, as Matthias told you already, at the end of charging you'll see that voltage.

See that cable resistance like a current limiting thing, because bulk is already by definition a CC (constant current) process, while the float is a CV (constant voltage) process. And for sure during that CV stage the final current will be very small and the voltage drop will be likewise very small, so you'll see that voltage anyway... You just don't have enough patience for it... :-))) Just kidding...

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bjornm avatar image bjornm commented ·
Not LFP (lithium). Usually it is bulk to a specific voltage, then straight to float. No absorption. Then it is an advantage to have a charge controller that stops the bulk at the correct voltage. And this impossible to really know without battery voltage sense cables. It will be roughly right without those cables, and maybe that is good enough for some people.
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snoobler avatar image snoobler bjornm commented ·

Your statement is completely incorrect. LFP charges tend to have short absorption times, not zero.

Most cells spec a charge termination criteria of 3.65V/cell and 0.05C, so an absorption period is required for typical 0.2-0.5C charges UNLESS you're charging at 0.05C.

Behavior really depends on your target voltage and charge rate. Most LFP require some time at absorption to ensure the cells stay at elevated levels for passive balancing. Victron recommends at least 2 hours per month. Battleborn recommends something similar.

The net effect of a lack of voltage sense leads is that your battery will take slightly longer to charge (a few minutes) to the correct voltage than if you had sense wires due to the early current taper induced. If you're charging to 3.50 or greater, set a 10 minute absorption period and/or a tail current of 0.05C (add your loads as needed), and you'll be close enough to 100% that you won't be able to measure the difference.

Since you're only charging to to 3.45V/cell, you actually need an absorption period for certain. If you run to 3.45V and then cut charge, you will be at a lower SoC. Set at least a 30-60 minute absorption period, and you'll hit full. If you're already starting at a low C rate, you may be able to get away with a shorter absorption period.

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bjornm avatar image bjornm snoobler commented ·

"Completely incorrect", that's a bit harsh. ;-) Some people use absorption for LFP, some don't. It's a personal preference.

Standard charge to 3.65 V is for EVs, not solar. Most solar installations use between 3.45 and 3.55 V to be more gentle with the cells.

I'm charging at 0.1C at best, and often lower than that. So I pick a relatively low voltage of 3.45 V because at 0.1C the pack will get to approx 97% SOC (good enough), and for lower charge rates it will get to 100% or even slightly over charged. This is the setup I've used with my current charger, because it doesn't have a setting for current tapering. If I get the Victron I might go to 3.50 or 3.55 V with current tapering, so that is a good suggestion from you. Then the lack of voltage sense cables is less of an issue. Need to think it through first though.

BMSes are configurable for when they start balancing. Just set it a bit lower than your target voltage.

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djsmiley avatar image
djsmiley answered ·

If you use a Victron (Smart)shunt you can use that as voltage reference. That makes more sense and gives much more useable info (SOC) than having dedicated voltage sense wires to compensate for voltage drop.
You can't use a fixed offset, since voltage drop depends on the current and is not fixed.

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