Solar: Dual battery with PWM Vs. Single with MPPT?

[Polly's Kettle]

So, I've been researching the right spec to install solar on my boat.

I've come to one final hurdle, and hope to tap other' experience.

It seems that whilst MPPT is accepted as a better solar controller system, with reasonable costs now, there is no MPPT controller that can do two battery banks.

So the choice seems to be to have both the house and engine batteries charged, but use PWM, or just do the house with MPPT.

Does anyone have any useful experience to help me make this choice?

 

[VicS]

So, I've been researching the right spec to install solar on my boat.

I've come to one final hurdle, and hope to tap other' experience.

It seems that whilst MPPT is accepted as a better solar controller system, with reasonable costs now, there is no MPPT controller that can do two battery banks.

So the choice seems to be to have both the house and engine batteries charged, but use PWM, or just do the house with MPPT.

Does anyone have any useful experience to help me make this choice?

You don't provide any details of your system. Especially size of proposed solar installation or details of existing charge distribution

General wisdom seems to be that there is little to gain by using MPPT over PWM for a small solar installation.

Maybe an MPPT controller can be used in conjunction with a VSR based charge distribution system

The engine alternator should keep the engine start battery charged anyway .

 

[Polly's Kettle]

Ah - my proposed installation is indeed small - 120watts, from a single panel.
Many thanks.

 

[macd]

The engine alternator should keep the engine start battery charged anyway .

+1
If the boat is used regularly, the starter battery needs next-to-no charging, and the alternator will easily take care of that. If used only occasionally -- say only every couple of months or more -- it may be a different story.

P.S. Just noticed that there's a somewhat related thread here: http://www.ybw.com/forums/showthread.php?459520-What-gauge-wiring-to-Solar-Panels

(Is it just me who finds patently practical threads appearing on S'butt just a little puzzling?)

 

[vyv_cox]

I have 125 watts of solar panels (two in parallel) into a Sunworks PWM controller for two battery banks. At this time of year in Greece we are self sufficient for electricity, running a fridge 24/7. Later in the year we need to run the engine occasionally to keep volts up. The best thing we did was to add a battery to the domestic bank, now 3 x 110 Ah, a vast improvement over the previous 2 x 110Ah. My starter battery is a Red Flash 1100 that fits in the engine compartment, freeing up the space for the third domestic.

 

[billcole]

I think it's worth including a Voltage Sensitive Relay in the system whether you go for MPPT or PWM.

The VSR connects the two battery banks together whenever the voltage on either bank rises above about 13V, and isolates them if the voltage drops below about 12.7 volts.

This means you can hard wire your alternator to the engine start battery and your solar power to the house battery, and as soon as the engine battery gets back up to a decent voltage after starting then the alternator will automatically be used to charge the house battery. If you're sailing or moored up for a while then whenever the house battery is well topped then up the surplus solar power automatically goes to top up the starter battery.

No diodes, are involved so there are no problems with voltage drops across diodes losing power or causing the alternator to sense the wrong voltage.
You only need a single output solar regulator, so can choose whichever of the two types you prefer. My cheap ebay MPPT works well. but very occasionally gets confused and shuts itself off until it is reset by having the power disconnected, so I use it when on board, but swap over to a dumb PWM one when leaving the boat (in Greece) for extended periods in order to make sure the batteries aren't left uncharged by a tripped regulator.

 

[GrahamM376]

I have 125 watts of solar panels (two in parallel) into a Sunworks PWM controller for two battery banks. At this time of year in Greece we are self sufficient for electricity, running a fridge 24/7. Later in the year we need to run the engine occasionally to keep volts up. The best thing we did was to add a battery to the domestic bank, now 3 x 110 Ah, a vast improvement over the previous 2 x 110Ah. My starter battery is a Red Flash 1100 that fits in the engine compartment, freeing up the space for the third domestic.

What kind of boost charge voltage do you get from the Sunworks?

I have 200w solar charging 2 banks, each 2 x 6V Trojan T125, 240AH at 20 hr rate. These were fed though 2 regs., a Wellsee WS-MPPT30 and a nondescript PWM. Watching the NASA BM1s, the PWM always gave a better charge - until it packed up!

Now just using the Wellsee for both banks but its cut-off voltage is around 13.3v which isn't much use as the Trojans should get Bulk 14.8 and 13.5 float. Watching the BMs now, 13.3v charge amps keep dropping from 4.5 per bank to 0.2 amps. (Fridge switched off temporarily) Need something with higher output.

 

[jac]

You haven't explained what you're trying to achieve.

Is it to keep both banks full and defeat any self discharge when the boat is unattended for a period? Small panel via PWM to both banks is good idea.

Is it to recharge domestic bank over a working week and also keep the starter topped up - larger panel via PWM to both might be the way to go.

Is it to recharge the domestic bank daily so that the engine need only be run every couple of days whilst moving anyway and not for battey charging - Largest panel via MPPT is the way to go.

Assuming you are looking to pump in the maximum amphours you can in the shortest time possible then you do need to give some consideration to looking after the engine start battery. Over the summer it's probably fine as I assume it won't go more than 2-3 weeks withiout being used. AUtumn & Winter though may present additional challenges. a small panel ( 5-10W) should just about keep up with discharge in the middle of autumn / spring but may not do so over the darkest part of winter, especially if you're tucked into a sheltered corner of a boatyard! Shore power may be one solution or paralleling all the batteries ( if similar) and letting the solar panel trickle charge all of them.

 

[vyv_cox]

What kind of boost charge voltage do you get from the Sunworks?

At the moment I have 14.0 volts but I have seen 14.3 or 14.4.

 

[GrahamM376]

At the moment I have 14.0 volts but I have seen 14.3 or 14.4.

Thanks for that. I decided against that model as it only sends boost voltage to one bank, charges that and then the second one, no boost voltage though. May parallel my 2 banks permanently rather than by switching and just get a decent single output reg.

 

[gregcope]

My starter battery is a Red Flash 1100 that fits in the engine compartment, freeing up the space for the third domestic.

Vyv,

Can you give an over view on how you changed the wiring?

Did you just run a +ve between the engine battery and back of the off/1/2/both master switch which I assume is not near that area? I assume there is a -ve to use somewhere in the engine?

Considering doing the same, but wondering what the wiring needs to look like.

 

[vyv_cox]

Vyv,

Can you give an over view on how you changed the wiring?

Did you just run a +ve between the engine battery and back of the off/1/2/both master switch which I assume is not near that area? I assume there is a -ve to use somewhere in the engine?

Considering doing the same, but wondering what the wiring needs to look like.

Originally there were three batteries in a moulded bin, one starter, two domestic. Separate wires for starter and domestic to the 1-2-both switch. It was just a matter of replacing the cables when moving the starter battery, which sits upright alongside the gearbox. And a set of parallel cables to convert the third battery to domestic. All my earth wires go to a bolt on the engine/gearbox connection.

 

[gregcope]

Originally there were three batteries in a moulded bin, one starter, two domestic. Separate wires for starter and domestic to the 1-2-both switch. It was just a matter of replacing the cables when moving the starter battery, which sits upright alongside the gearbox. And a set of parallel cables to convert the third battery to domestic. All my earth wires go to a bolt on the engine/gearbox connection.

Thanks makes sense.

My batteries are under the port saloon berth, and the +ve cable should be more than long enough to reroute to the engine compartment from the 1-2-both switch. Although that might be an interesting "this should only 5 minutes", but actually takes 4 hours sort of jobs! Maybe one for winter!