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?
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.
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.
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.
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.
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.
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.
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.
@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.
Cheers.
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...
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.
"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.
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.
