I have tried different MPPT solar chargers, and they all seem to have the same defect in relation to Lithium-ion batteries. The algorithms seem to wait until the battery reaches its low voltage charge point, typically 13.2 Volts before the solar panels kick in. When the sun is directly overhead and solar panels have a 15 Amp capacity, the solar chargers do not supply the boat load, the load is supplied from the Lithium-ion batteries until such time as the low voltage charge point is reached. Does Victron have a MPPT charger that senses the available solar power and uses this to supply the load, rather than discharging the Lithium-ion batteries. By the time the batteries reach the low voltage charge point the sun is way over near the horizon, and the midday power available from the solar panels is not being utilized.
If you have set the float voltage at 13.2v this will happen.
Perhaps a screenshot of your mppt settings?
Victron MPPT chargers have a setting called rebulk voltage offset as explained below which can avoid this problem. The default float voltage is 13.5V and can be configured so once charged, this holds the battery at around 99% full and supplies loads.
Copied from the manual.
Re-bulk voltage offset
This setting sets the re-bulk voltage offset. This offset voltage is used to determine when a charge stage stops and the bulk stage starts again, i.e. the charge cycle resets and starts at the first charge stage again.
The re-bulk voltage is calculated by subtracting the re-bulk voltage offset from the lowest voltage setting (normally this is the float stage).
An example: If the re-bulk offset is set at 0.1V and the float voltage at 13.5V, the charge cycle will restart once the battery voltage drops below 13.4V (13.5 minus 0.1) for one minute.
What voltage are your panels?
Victron MPPT only starts charging when the panels are at Vbatt + 5V.
Thank you all for your replies. Unfortunately, the issue has nothing to do with Re-bulk settings, Vbatt +5v or the setting of the float voltage. Let’s assume the battery has reached full charge at 14.4 volts. Between 14.4 volts and the low voltage recharge point (let’s assume 13.4 volts) the battery supplies the base load (let’s assume 5 amps) even if the sun is directly overhead. This is a waste of available solar power. I believe MPPT solar chargers need a current shunt input so as it senses that there is a base load, looks at the current battery voltage, and then maintains that battery voltage using the solar array, and adjusts the solar output so as to maintain a zero-shunt current ie The solar array is putting out enough current to compensate for the discharge current. Example…battery is at 13.7v, base load is 5 amps and sun is directly overhead. Adjust solar array such that it’s 13.7 volts and supplies 5 amps of charge current, resulting in no discharge of the battery. All MPPT chargers exhibit the same basic problem.
Not really a problem if you look a little closer.
From my system today.
About 11:30 the battery reaches the batteries target bulk voltage of 14v.
Mppt goes to absorption, current reduces to maintain 14v.
About 11:50 after the programmed absorption time has elapsed, mppt changes over to float. Target float voltage is 13.35v.
It take about 30 minutes (with a inverter/fridge as load) for the battery voltage to finally drop down to the float voltage.
During that 30 minutes, the battery is powering both the inverter/fridge/other small loads.
The loss of solar input during this 30 minutes drains my battery by 1% Soc.
No harm to my battery Soc.
Once the mppt has switched to float it maintains that battery voltage/Soc
until there is no longer enough sun available.
I do see that the period between absorp and float cuts solar production to zero.
Though, once the mppt has switched to float, it maintains the battery voltage and Soc.
I understand your question now, a little unusual which is why we missed it.
Lets say that the battery has been charged to 14.4V and is full. If you now want the solar to just cover the loads and not use any battery power it would have to remain at a higher voltage to stop the battery discharging. That is bad news for the battery as lithium batteries do not like being held at 100%, ideally once at 100% they should be backed off to a voltage just below 100%. Then you are left with a battery that has to discharge some voltage. The trick is making this “float” voltage as high as possible, so 13.5V or 13.6V rather than 13.2V.
Thank you klim8skeptic for your comprehensive reply. I was concerned that the 14.4v - 13.2 v charge/discharge cycle will affect the 4000hr cycle life of the lithium. When I rang the manufacturers, they explained that the 4000hr cycle life only refers to 4000 cycles where the battery has had 4000 cycles at 50% capacity only. Having looked at your graphs can you please let me know which make and model of MPPT you are using. I am still a little bit skeptical about the cycle life and am still investigating other lithium battery manufacturers to ascertain exactly the recommended bulk/float and recharge points. Thanks again.
[quote=“Boomaroo, post:8, topic:32659, username:Boomaroo”]
Thank you pwfarnell for your reply. Please see my reply to klim8skeptic and I am still investigating what the manufacturers think about setting the recharge point to the float value rather than 13.2v…which seems to be the norm for lithium batteries.
Victron 100-50 with 760w of panels.
There’s also a Victron 75-50 with 600w of panels.
I would not worry about the cycle life.
Degradation due to high temps, high charge voltages, or low voltages due to very deep discharges, will wear the battery out well before the cycle life is up.
As for recommended Charge voltages, use the manufactures charge specs.
Do be aware the some battery manufacturers use different terminology for bulk-absorption-float.
Time the mppt spends in absorption is to ensure the cells are balanced.
Shorter absorp time for batteries fully charged regularly, longer absorp time for batteries charged occasionally.
Older Victron lithium batteries floated at 13.5V, newer ones at 13.7V really minimises this effect holding them at 99 to 99.5% and supplying loads. But as said elsewhere go by the battery manufacturers datasheets.
Hello pwfarnell and klim8skeptic. I have had 3 all in one dc-dc/MPPT chargers. 2 failed after nearby lightning strikes (Enerdrive - mppt input only) and my current one I am not happy with (Itech World). So…after reading all your comments and doing a heap of research I think I will go down the following path. I have 1 x 200w solar panel which I will connect to a Victron 100/30 or 75/15. I will invest in another 2 x 100w solar panels (they will fit the mounting area). Unfortunately can’t seem to find equivalent 100w panels with same voc as the 200w, so will wire the 100’s in parallel and feed them to another 100/30 or 75/15. The 2 Victron’s will then both feed the same battery bank with the same settings on each one. I will also get a Victron Orion XS 12/12 50 Amp dc-dc battery charger to run off the 2 engines on my catamaran, and set this up with the same settings as the mppt’s. I currently have a Blue Smart Charger 240v/12v 30 Amp unit configured as a power supply, which I can switch into the dc-dc charger when at the marina. I’ll let you know the outcome. Will configure the low point charge volts at 0.1v below the float V (probably set to 13.5v). Just now want to get rid of the all-in-one unit, as speaking to other boat owners they seem to have less problems using individual units. Cheers for now