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Correct sizing of solar panel for float charging

I am going to add a solar panel to my boat just for providing float charge to my starter battery (12v 240ah) and bow thruster battery (24v 240ah). I will use a Smartsolar 75/15 controller and an Orion 12/24 to step up to the 24v battery.

My question is this: what current do I need the panel to provide to enable float charging of these batteries? I'm think a 115w panel will provide typically 8 amps in the sun but rather less most of the day and, of course, nothing at night.

Not worried about bulk charging - the alternator will do that and these batteries will only discharge at start or when the alternator is running



rob-dean asked
kevgermany answered ·

2 Answers

Battery in floating even if the load higher then the solar power

Dear All,

Can someone have an idea why the Multiplus II stay in float mode even if the required power for the load is higher then the solar power. Normally, battery power must be combined with solar power to absorb the load power.

Best regards

nostromo asked
nostromo commented ·

2 Answers

Charger says "Float" even though it's below Float voltage.
I really feel like I should be understanding this better than I am. I already read a bunch of posts here. I understand how "Absorption/Float/Bulk" works in the sense of charging from zero to 100. But I'm confused about it right now, as it discharges and then charges up again from 65%.

I'm doing an initial slow cycling of my batteries to get them ready. And I'm a little confused by what I'm seeing here. Currently sitting at ~53.5 volts, below the float voltage, and it's charging my battery at my specified rate. Why does it still say float? There's something about Float/Absorption/Bulk that I'm not getting. Wouldn't this be a bulk phase since the voltage is lower?

Mostly, I'm just confused why it says "float" when it's charging at the bulk charge rate.


  • Victron Multiplus
  • Absorption Voltage: 56.8v
  • Float Voltage: 54.0v
  • Battery: LiFePO4
    • 300AH
    • 48v



The voltage was previously down to 52.15v, and still said "float". That doesnt seem right.


coreyzev asked
goncalo answered ·

3 Answers

Multiplus 2 with ESS Assistant shows strange Float charge behaviour...

I use the following setup: ESS System with a Multiplus II 48V, CerboGX and a Lithium NMC battery that is protected by a built in BMS. The Multiplus does not communicate with the battery. Charging and discharging is controled by voltage.

I am currently testing the following settings:

TAB: General

System frequency 50Hz

Shore current 13.0 A

Overruled by remote unchecked

Dynamic current limiter unchecked

External current sensor connected (see manual) unchecked

State of charge when Bulk finished 100.0 %

Battery capacity 50 Ah

Charge efficiency 0.95

TAB: Grid

Country / grid code standard Germany: VDE-AR-N 4105:2018-11, internal NS protection

LOM detection AC input 1 Type B (grid code compliant)

rise-in-voltage protection U> 253.0 V

start network service HF treshold 50.20 Hz

P(f>) droop 5.00 %

Use Aux1 as disable FeedIn signal checked

Maximum AC current for charge or feed in 100.0 %

Reactive power regulation Use a fixed Cos Phi

Filter time for reactive power 3.3 s

Cos phi at point 1 1.00

TAB: Inverter

PowerAssist unchecked

Inverter output voltage 230 V

Inverter DC shut-down voltage 39.20 V

Inverter DC restart voltage 45.60 V

Low DC alarm level 45.60 V

Do not restart after short-circuit (VDE 2510-2 safety) unchecked

enable AES unchecked

TAB: Charger

Enable charger checked

Weak AC input unchecked

Stop after excessive bulk unchecked

Lithium batteries checked

Configured for VE.Bus BMS unchecked

Charge curve Fixed

Absorption voltage 53.00 V

Float voltage 52.98 V

Charge current 35 A

Repeated absorption time 0.25 Hr

Repeated absorption interval 45.00 Days

Absorption time 1 Hr

TAB: Virtual switch

TAB: Usage

Virtual switch usage Do not use VS

TAB: Assistants

TAB: Assistant Configuration

ESS (Energy Storage System) (size:962)

*) System uses LiFePo4 with other type BMS

(This can be either a BMS connected via CAN bus or a BMS system in which the

batteries are protected from high/low cell voltages by external equipment.)

*) The battery capacity of the system is 50 Ah.

*) Sustain voltage 44.80 V.

*) Cut off voltage for a discharge current of:

0.005 C= 47.80 V

0.25 C= 47.20 V

0.7 C= 46.60 V

2 C= 46.10 V

*) Inverting is allowed again when voltage rises 1.20 V above cut-off(0).

*) Relevant VEConfigure settings:

- Battery capacity 50 Ah.

- PowerAssist unchecked

- Lithium batteries checked

- Dynamic current limiter unchecked

- Storage mode unchecked

Total size of all assistants including the required

(hidden) system assistants is: 1021

It works pretty well...

Now my question:

When there is enough solar to power home consumption the rest is used to recharge the battery. In bulk charge it all works fine, as soon as the absorption phase ends, the inverter discharges a little bit to reach the float voltage of 52.98 V. But then the inverter stays on (green light is on) all the time.... even though there is enough solar power to power the house and therefore the inverter not needed.

How do I have to change the settings in order to prevent this behaviour?

Or is there a way to completely disable float charge, as it is not useful for my type of Lithium-Ion battery?

Thanks in advance for your feedback and help!

save1power asked
Alexandra edited ·

2 Answers

Pylontech : pas de tirage et multiplus : no float


J'ai multiplus 3000kva/48, avec 4 pylontech us2000C en ESS.

Pourquoi j'arrive pas à utiliser l'énergie des batteries ?

En paramétrage, j'ai utilisé :


Qui est identique en paramétrage victron mais synthétisé.

Et le multiplus ne passe jamais en float. J'ai un retour de batterie en : "ralentir la charge".

J'espère avoir donné toutes les informations.

Merci de votre aide.

vins asked
vins answered ·

4 Answers

MultiPlus-II 12/3000/120-50 120/240V SOC when in float
I recently completed installation on my Victron MultiPlus-II 12/3000/120-50 120/240V Charger/Inverter along with a SmartShunt 500A and 300AH of 12 LiPO batteries and Cerbo GX in my RV. I have not gotten a chance to install the PV system yet. Everything works great, but I am trying to tweak my settings. What I am noticing is that when I am in Bulk and Absorption, my batteries charge to 100% SOC. Once it goes to float, the parasitic drain of the 12v system (approx 23w) slowly drains the battery SOC down.. I haven't see how low, but I was wondering is this normal? Shouldn't the charger keep the battery at near 100%? Is it possible I didn't correctly configure my SOC correctly on the shunt? If so what is the typical conditions to zero out the SOC? Is this maybe how the system protects the longevity of the batteries? Sorry for the rookie questions, just in all my research I haven't seen anything about this. I just want my system to be charged to 100% when in use so that if we lose or need power I'm not starting at 80% SOC. Any suggestion would be appreciated.

Absorption Set at 14.2v

Float set to 13.6v

Smart shunt set to 14.1V



paul-martin asked
paul-martin answered ·

2 Answers

Absorption to Float triggered at 40% or 60% of SoC (Multi + BMS + VE Smart Lithium LiFePo4 300ah Battery)

Multiplus is going from ABSORPTION to FLOAT charging state when the SoC is at 40% o 60%, which should actually been triggered when the battery is fully charge, if I'm not wrong.


Multiplus settings are the recommended ones for Lithium batteries. So floating is at 13.5v and absorption is at 14.2v.



- Could it be the Absorption time?

Any help would be really appreciate!!! thanks in advance to this great community.

esteban-moreno asked
klim8skeptic edited ·

1 Answer

Multiplus float voltage not stable causing battery in charging/discharging cycle

I have Multiplus 3000 and two battle born Lithium batteries in parallel. The trailer is plugged into my house’s 30amp. The float voltage is set to 13.5v. The Smart Shunt is connected to the batteries terminals and the Smart Dongle is plugged into the buses. When Multiplus is in float mode and DC load is running, I noticed sometimes the power was pulled from the batteries and sometimes the Multiplus charged the batteries. The voltage is bouncing up and down. I set the Multiplus to charger only, it’s the same. Is it normal the batteries got charged/discharged cycle in float mode and the voltage bounces? Should I disconnect the battery when plugged in to avoid cycling the battery?


joelimvp asked
joelimvp answered ·

2 Answers

MPPT 100/30 float issue

Hi all,

I've had a look through the masses of previous questions and unfortunately couldn't find any that were quite the same as mine.

Recently I have noticed that my SmartSolar 100/30 completes it's bulk and absorption stages and enters float mode. When in float it will match any current draw from the battery for roughly an hour and then will suddenly no longer keep matching the battery and it will then start dropping down from 100%.
It says it is in float mode, there is plenty of sunlight and panel voltage present, however it doesn't output any charge to the battery. No other chargers connected to the battery.

I have the float voltage set to 15.8V and even when the loads connected to the battery increases it does not kick it back into bulk.

I then have to reset the controller by disconnecting the solar, then battery and reconnecting, which it will then it go back into bulk mode, absorption mode and then eventually into float again.

Has anyone else experienced this issue? Is it potentially a firmware issue?






ksmith asked
linuxhiker answered ·

9 Answers

150/70 Switching back to Bulk less than 10 mins after being in Float

Installed my new Victron 150/70 and programmed the battery manufacturers settings. Turned on the panels to charge the batteries and the MPPT started to charge in bulk for 5hrs as it should than went to absorption for 1hrs than float. After less than 10 mins in float the MPPT went back into bulk, but was not producing any current. There was no load on the batteries and it was early afternoon. Have I missed a setting or is there something wrong with the MPPT?




islander-off-grid asked
islander-off-grid commented ·

1 Answer

ESS bulk restart voltage causing stuck in float

I try not to be too hard on my Lifepo4 batteries so I charge to 56v and float at 53.6v. This worked fine with the MPPT charge controllers, but once I added ESS the problems began. The issue is the bulk reset voltage. On the charge controllers this is float -0.4v so bulk restarted at 53.2v, or around 80% battery capacity which was fine.

With the ESS system, Victron in their wisdom have chosen to set the bulk reset at float -0.8v

Why different ?

This means bulk does not restart until 52.8v. With the flat profile of Lifepo4 this is around 63% battery capacity, which is way too low. Until this point it just sits in float. If you heavily load the system to drop the voltage then it changes over to bulk without any problems, but sitting with a low load you lose a lot of charge time.

Does anyone know a way to adjust this bulk reset voltage ? I know I could float at a higher voltage but this seems a poor solution to the problem.

muddy1 asked
Leslieanne commented ·

0 Answers

DC to DC 12/12 30 entering float mode too early on my Ford transit conversion...HELP!

I have a Victron Orion tr 12/12-30 connected to the starter battery in my Ford Transit camper. It seems to be working, but keeps going into absorption and float way too early.

When I start the engine and drive away, the unit goes into absorption for approx 10 mins and then floats. It does keep my battery % at whatever it was when we left but wont increase the %.

I have 1 x 260ah agm battery. I use a victron MPPT, a BMV and a battery protect.

I have attached screen shots, and would love to know why it doesn't seem to be working properly!

I am in Scotland and could do with the power. There is f'all sun!







peevichpc asked
kevgermany answered ·

1 Answer

Bluesolar 75/50 gets to float voltage too early.

Seen some with similar problem but not seeing many answers. Please help. Trying to improve SOC during daylight hours. Have Smartshunt onto 600Ah Lithium. Po4. Come the afternoon, solar producing close to 50Amps. The voltage moves increasingly higher and then goes into Float. Float set at 14.2v. Trying to keep voltage low and Amps high to increase SOC. Is the charger regulating the max. Output by increasing the charge voltage? When I turn the charger off for a few minutes, voltage regulates to below 13.8 and charger goes into Bulk mode with higher charging current obviously. Running VE Bluetooth dongle and Smartshunt networked

wisey asked
klim8skeptic commented ·

5 Answers

Boondocking with BMV-712: SOC error? Best practices?

I have the following setup: smaller 2015 Lance travel trailer with 2ea 12V lead acid flooded batteries in parallel, BMV-712 and the standard Honda 2kW generator. The trailer's battery charger seems modern (3-stage charging, lots of amps, etc.).

I plan to boon dock (no shore power) regularly and would like to tune the BMV-712 to help keep the batteries charged, healthy, and to know when and how long I should run the generator.

From research, I know I should always keep the batteries above 50% capacity to avoid premature replacement costs. Also, without many hours of charging (e.g., plugged in or on solar), I cannot expect battery capacity to rise above 90% while boon docking.

When charging from, for example, ~60%, the charger will be in the "bulk" mode with high current, and rising voltage. This charge mode is the quickest, but only gets to 80% capacity.

Then the charge mode changes to "absorption" where the charger voltage is held high (14.6v in my case) and then the amps fall off toward ~2amps (in my case) when the charge mode changes to "float" at which the battery capacity is at 90%. To charge from 90% to 100% capacity would take many hours and isn't reasonable while boon docking.

Therefore, while boon docking, I should expect to cycle the battery between 50% and 90% depending on loads and generator run-times.

However, AFAIK, the BMV-712 is designed to treat the 90% charge state as "100% SOC" (?) The BMV-712 decides that when the voltage is the high "absorption" mode, and then the current drops below a "tail current", it proclaims SOC = 100%. But in reality, the battery is at 90% capacity at that moment (i.e., at the end of the "absorption" mode).

If starting from a full-charge (i.e., batteries have been plugged in for a day), the SOC value will be fairly accurate. However, if not starting from a true full-charge (allowing the BMV-712 to auto-sync the 100% SOC value at the end of the absorption mode), the reported SOC value will be 10% lower than reality. So the accuracy of the reported SOC value depends on whether the previous charge completed the float mode? Is this true?

If that's true, then I must tweak other settings to make the SOC value match reality or add-in some fudge-factor or work-around.

I could:

  • tell the BMV-712 that the "battery capacity" is 10% lower than the battery specification.
  • set my "SOC relay (charge floor)" to 60% instead of 50%, and mentally never let the reported SOC value fall below 60%

Is this a known problem with the BMV-712 SOC value? What do others do to work-around this inaccuracy?

Do other boon dockers have tips on using the BMV-712 to stay confident that the generator is running for the right amount of time?


goodguy5000hd asked
captn-mike answered ·

2 Answers

Battery is not charging anymore?!


I am not sure if it's connected to an update I did, but my Battery is not charging anymore. The Smartsolar is showing me Volt from the solarpanel but no Watt is coming in. In the history the battery minimum is always 0... After reconnecting everything and checking the cables it stays on bulk for 2 Seconds and changing to floating. I will ad some Screenshots, maybe someone can help me???

jonny asked
jonny commented ·

1 Answer


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