Solar mppt to battery cable .

[Greg & Jax]

Hi all .

Looking for a bit of advice .

I've recently bought a solar kit from bimble , 2 295 w panels and 60 amp mppt ,  but the ideal  place to put the mppt is about 3 mtr away from the batteries .

What would be the right size of cable to use ? 

Regards Greg. 

[Jen-in-Wellies]

What make/model of mppt do you have? Some have an option of a separate battery voltage sensing wire input. This will make any voltage drop in the delivery cables less of an issue and compensate for any drop seen. In answer to your question; as thick a cable as you can get away with. 600W of panels will be 50A at full blast, but that is unlikely in the UK. Use a voltage drop calculator. This suggests 25mm^2 cable will get the voltage drop at 50A down to 0.462V.

You say the ideal spot for the controller is 3m away. A non ideal spot would save a lot of money in cable, especially if the two panels are wired in series to boost the voltage and drop the current between panels and controller.

Jen

[GUMPY]

Cable between the MPPT and the batteries should be the largest you can get into the MPPT terminals on my 40 amp it's 16sqmm.

 

[Jen-in-Wellies]

2 minutes ago, Loddon said:

Cable between the MPPT and the batteries should be the largest you can get into the MPPT terminals on my 40 amp it's 16sqmm.

 

Good point. Maybe go from 16mm^2 to 25 once beyond the MPPT as an entire run of 3m would increase the voltage drop to up to 0.75V at 50A.

[Greg & Jax]

Hi .

The mppt is a epever AN 60 amp.

Because of its size it will get either wet or banged anywhere else in the engine room .

Thank you for the quick reply,s .

Greg .

[Jen-in-Wellies]

An option could be to use existing large diameter cables. If some 25, or 50mm^2, or whatever cables are going to domestic electrics, or an inverter and are passing closer to the location for the MPPT, then you could connect in to them. This is going to be downstream of an isolation switch, so not an option if you want solar in to the batteries with the house electrics turned off. Depends how you use the boat. Also need a switch in the input from the panels. Many MPPT controllers don't like being powered up with batteries disconnected. You need to remember to turn off the panels before throwing the main isolator switch. I have mine wired this way, but this boat is lived on full time. If it is abandoned with the panels trickle charging the batts for months on end, then this isn't a good approach and you want the panels directly wired to the batteries with a fuse, but no switch. BSS allows this. In that case, then a separate run of large cross section wiring will be needed.

I can't find the manual for your controller from your description with any certainty, so can't help on questions of voltage and temperature sensing.

Jen

 

Edited to add: Just noticed a mistake that means that all voltage drop calculations I've shown above are double what they should be. Oops. Apologies. Acceptable wire sizes go down as a result. 16mm^2 probably fine for 3m away.

Edited November 27, 2021 by Jen-in-Wellies

[Greg & Jax]

Hi all .

Thanks for the your reply,s ,ideas. 

Regards Greg. 

 

[Detling]

Whatever the volt drop is it will fool the controller into thinking the batteries are fuller than they are, and thus going into float earlier than it should. The higher the volt drop the earlier this will happen so the 0.75 volt drop mentioned above would cause the controller to stop charging the batteries when they are only about 80% full which is not desirable,  continually udercharging is the fastest way to kill batteries. You need at least 16mm and preferably 25mm cable for that length. Use the volt drop calculator at https://www.12voltplanet.co.uk/voltage-drop-calculator.html to help you, but at 3 meters you are going to struggle to get the batteries over 90% without changing a lot of settings on the controller to make the float voltage high, but then in summer that will overcharge the batteries.

I would put the panels in parallel not series as this will reduce the impact of a bit of shade on one panel, even a rope across a panel can halve its current, and if the panels are in series that will halve the total available power, in parallel you would only lose a quarter. People say that in series is better for low light but a solar panel reaches 90% volt output with only a few milliamps available so naff all power really. the EPever controller will not track the maximum power point until 1.5 amps comes out of it into the batteries, below that it runs in PWM mode chopping off half the available power, parallel wiring gets the thing working properly earlier. (other MPPT controllers don't have this problem but EPever / Tracer and it's rebranded versions all suffer from this 'feature', other than that they are good.)

[Greg & Jax]

Thank you Detling .

After reading all the reply,s .

I'm going to have a rethink of where I can put the controller before the final fitting .

Thank you for the input .

[George and Dragon]

18 hours ago, Detling said:

Whatever the volt drop is it will fool the controller into thinking the batteries are fuller than they are, and thus going into float earlier than it should. The higher the volt drop the earlier this will happen so the 0.75 volt drop mentioned above would cause the controller to stop charging the batteries when they are only about 80% full which is not desirable,  continually udercharging is the fastest way to kill batteries.

This is true so far as it goes. Volt drop is proportional to current. If you're putting 50A in then the batteries are a long way from fully charged. If you're only putting in 5A the volt drop is reduced by a factor of 10 so that theoretical 750mV is now just 75mV. Of course if you're still drawing 45A into appliances the volt drop at the MPPT won't change but the danger of failing to fully charge the batteries because of volt drop is perhaps a little overstated.

[Greg & Jax]

Thank you for your thoughts on this .

I'm very new to this solar stuff  and so doing a DIY fit is a bit of a minefield. 

Regards Greg. 

[Jen-in-Wellies]

On 27/11/2021 at 08:52, Greg & Jax said:

The mppt is a epever AN 60 amp.

Is it this one? It is hard to tell from the very contracted model number you quote, but this is the only one claimed to be able to handle 60A. It is easier to help if you tell us exactly what you have. If it is, then according to the manual, it does include a battery sense connection which can be used to sense the battery voltage and compensate for the varying voltage drops in the cables at different currents. It is very worth while using this, whatever the cable size you finally choose, given the long length. Also worth buying the optional remote temperature sensor and attaching that to the batteries, as charging voltage is ideally adjusted to suit the battery temperature and again, your batteries are a long way from the controller in a different compartment, with different temperatures.

Worth talking to Bimble about voltage sensing. The manual says on the 21st page:

Quote

 Connect Remote Battery Voltage Sensor (Model:RVBS300B3.81)
Connect the remote battery voltage sensor cable to the interface ➀and connect the other end
to the battery terminals

However the part number is for the temperature sensor, so I think this is a mistake and it should read Battery Temperature Sensor, not Battery Voltage Sensor, especially as the next line is about remote battery voltage sensor cable. For the battery voltage sensing cable, in the absence of better advice from Bimble, I'd use a small gauge, 0.5mm^2 or so, red and black wires to the battery terminals, with a 1A fuse in the positive line near the battery to protect the wires.These wires take a miniscule current independent of the charge current and are just used to check the battery voltage, so are almost unaffected by a wire length of 3m.

Jen

 

Edited November 28, 2021 by Jen-in-Wellies

[Murflynn]

2 hours ago, George and Dragon said:

.....................   the danger of failing to fully charge the batteries because of volt drop is perhaps a little overstated.

 absolutely.

my 75A MPPT is about 3m away from the batteries, I have 300W solar. 

my batteries show a constant 13.9V when the MPPT indicator LED indicates charging has gone into float mode.

the cables are 6sq.mm.

[Greg & Jax]

Yes that the one jen .

I also have the remote temp sensor

Regards Greg. 

Edited November 28, 2021 by Greg & Jax
Added

[Jen-in-Wellies]

40 minutes ago, Murflynn said:

 absolutely.

my 75A MPPT is about 3m away from the batteries, I have 300W solar. 

my batteries show a constant 13.9V when the MPPT indicator LED indicates charging has gone into float mode.

the cables are 6sq.mm.

In float mode, the current is low anyway, only a few amps at most, so the voltage drop will be correspondingly low. It is only in bulk charge of a depleted battery bank that the voltages could be lower than expected, assuming no compensation is applied.

Jen

[George and Dragon]

2 hours ago, Greg & Jax said:

Thank you for your thoughts on this .

I'm very new to this solar stuff  and so doing a DIY fit is a bit of a minefield. 

Regards Greg. 

I'm very happy to be made to think about various aspects of this even though I don't have plans (yet?) to fit PVs. 

[Murflynn]

22 hours ago, Jen-in-Wellies said:

In float mode, the current is low anyway, only a few amps at most, so the voltage drop will be correspondingly low. It is only in bulk charge of a depleted battery bank that the voltages could be lower than expected, assuming no compensation is applied.

Jen

 

agree, but the consequence of thin cables will be a delay to reaching float mode.  

I guess that many folk rely on their solar to keep the batteries charged up while the boat is unoccupied, not to charge them from a seriously depleted condition.

'orses fer coarses.

[Tony Brooks]

1 hour ago, Murflynn said:

 

agree, but the consequence of thin cables will be a delay to reaching float mode.  

I guess that many folk rely on their solar to keep the batteries charged up while the boat is unoccupied, not to charge them from a seriously depleted condition.

'orses fer coarses.

 

I am not so sure about that if the controllers are using adaptive charging because that seems to time the voltage rise and make a guess as to the time the drop to float should occur. That usually seems to cause an early drop to float.

[cuthound]

To fit large cables into smaller terminations, use cable lugs with reducing pins, like these.

 

 

[Greg & Jax]

Thank you guys .