Battery questions for trolling boat conversion

samppa · September 2014

Hello!

So it seems I'm getting an oldish "trolling" boat (not sure about the proper english name for these..).. it's 6m long and has a hull speed of about 6 knots. Right now it has an old wikström diesel engine from around 1960. Engine is rated for 10hp.. probably doesn't give that much power..

So I'm intrigued about electric conversion of this boat. 4 or 5 kw electric engine should do.

First is battery stage. Weight is not so much an issue, so I think I can go with AGM. No need for more expensive lifepo.
Say I want to have 1-2h of battery range. So that after that, I would need to start a generator, when the batteries are around 50% SOC.

Se let's say 4kwh from batteries.. that's about 42 AH@96V. I could use 8 12v AGM batteries in series. Or 84AH@48V.. I haven't yet done much research on what kind of engine to get, but seems that most of them are rated around 72-96v.

Now my question: is it in any way possible to fit enough solar panels to the boat to charge this kind of batteries?

Most of the time the boat is used as follows:
- on friday I ride it to my cottage, about 20min
- during sat, a little fishin trip, maybe 1 h
- on sunday I ride it back, about 20min

It sits at the pier for the rest of the week. Over the weekend it sits at the pier at cottage. Only summer use. I'm in scandinavia, so summer solar insolation is plenty.

Hmm, or should I rethink this so that batteries will only be enough for this 20min ride, and use a genny for longer trips?

stephendv · September 2014

Re: Battery questions for trolling boat conversion

samppa wrote: »

Se let's say 4kwh from batteries.. that's about 42 AH@96V. I could use 8 12v AGM batteries in series. Or 84AH@48V.. I haven't yet done much research on what kind of engine to get, but seems that most of them are rated around 72-96v.

If you want to take 4kWh from the batteries, then you'd need double that in battery capacity so as not to discharge below 50%. I know a few people use flooded lead acid forklift batteries in narrow boats on UK canals. They're more forgiving and less expensive than AGM.... but if you're traveling over rough water then probably better to go with AGM or Gel.

So you'd need 84Ah + 20% for inefficiencies @ 96V, so about 100Ah batteries. To charge this, you'd need to replace the 4kWh you took out, using this map: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php# you can see how many kWh you produce every day for a given array size. Choosing Olso and a 1.5kW array at 35 degrees (and a loss of 30%), will generate just over 4kWh per day from April to July. You can play around with the variables to see what works best.
Also, if this is only for use in summer then it may be worthwhile splitting the array into 2 facing south-east and south-west to give you more energy during those long summer days. You may be able to scrape by with 6 x 240W poly panels, but you'd be better off with closer to 8.

What's the electric situation at the cottage? If you have solar power there too, then it may be worthwhile to think about sharing the energy from the panels. So when the boat is there it contributes to the cottage's energy or visa/versa, it can use the cottage's system to charge the batteries.

samppa · September 2014

Re: Battery questions for trolling boat conversion

Oh yeah, that's right, I didn't count the 50% discharge.. Problem here is that it's quite impossible to know exactly how much power that boat will need until it's been converted. Could be it takes 5kw to reach hull speed, or could it only takes 2kw to reach it..

I'm afraid there's no way to fit that many panels on to that boat..
Say I need a 1,5kW array to generate 4kWh per day.
What if the boat sits still for 4-5 days? So I have 4-5 days to charge to batteries back up? Do I still need a 1,5kW array, or how does it change?

There's solar at the cottage, only about 500w array. But I could expand that so I'd have power there to charge boat batteries too.

stephendv · September 2014

Re: Battery questions for trolling boat conversion

samppa wrote: »

I'm afraid there's no way to fit that many panels on to that boat..
Say I need a 1,5kW array to generate 4kWh per day.
What if the boat sits still for 4-5 days? So I have 4-5 days to charge to batteries back up? Do I still need a 1,5kW array, or how does it change?

It changes with simple maths

if you generate 4kWh in 1 day, then in 2 days you generate 8kWh... there might a tiny loss overnight, but nothing to be concerned about.

samppa wrote: »

There's solar at the cottage, only about 500w array. But I could expand that so I'd have power there to charge boat batteries too.

Yeah, that's the route I'd consider, get a bigger array where you can make the most use out of it and then charge the boat while docked. You can always use the generator to charge the boat batts if you need to use it, but batts aren't charged. The generator will give you a lot of flexibility, so if you wanted you could only install a fraction of the solar panels and supplement the rest with the generator.

samppa · September 2014

Re: Battery questions for trolling boat conversion

So if 1,5kW array will generate 4kwh / day, then 400w array would generate around 1 kwh/day. In 4 days, thats 4kWh.
Would this work? Or will I run into some other problems, such as not having large enough charge current for batteries?

The dock where the boat is stored has no shore power. And it will be stored there most weeks.

Another question: what size generator do I need? Worst case scenario: batts going flat, and need to use 4kw motor - do I need at least a 4kw generator?
If so, is a 4kw genny good for charging this size of battery bank?

oil pan 4 · September 2014

Re: Battery questions for trolling boat conversion

Find your charger before looking for the generator.
Then size the generator based on that.

BB. · September 2014

Re: Battery questions for trolling boat conversion

In general, for solar we use 5% to 13% rate of charge (based on 20 Hour Battery rating). 5% will work for multi-day charging, and I would not go any lower. Some fork lift and other lead acid deep cycle batteries recommend 10% minimum rate of charge.

For full time off grid use, 10-13%+ rate of charge makes things a lot easier to manage with the battery bank (get it fully/completely/properly recharged during normal day to day use).

One other thing to watch for--At least in the US, when you look at >48 volt battery banks, the number of charge controllers (brands/models) that can support that voltage drops a lot. Similarly, your AC battery charger options may be reduced too.

Note that you can put two 48 volt chargers in series on a 96 volt battery bank (or variations of that)--But there are some issues that can make the a bit difficult (one charger sees 0-48 volts, the second sees 48-96 volts, so you can have "shifted ground levels" between controllers--It can mess up communications circuits that use common grounds).

If you (for example) have a 48 volt @ 200 AH battery bank (or 10 amp rate of charge):

200 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 766 Watt minimum array (based on battery bank capacity and 5% rate of charge)

Another note--If you are in/near Finland, winter vs summer orientation of your solar panels will be a big factor in the amount of power you harvest per day. Using a fixed array and the Solar Electricity Handbook:

[h=3]Helsinki
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a horizontal surface:

Jan
Feb
Mar
Apr
May
Jun

0.32

1.10

2.44

3.96

5.41

5.63

Jul
Aug
Sep
Oct
Nov
Dec

5.40

4.09

2.54

1.18

0.50

0.20

[h=3]Helsinki
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a solar panel set at a 30° angle (from vertical):
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun

0.85

2.13

3.54

4.48

4.99

4.76

Jul
Aug
Sep
Oct
Nov
Dec

4.72

4.12

3.27

1.94

1.23

0.63

[h=3]Helsinki
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a solar panel set at a 15° angle (from vertical):
(Optimal winter settings)

Jan
Feb
Mar
Apr
May
Jun

0.88

2.13

3.39

4.06

4.40

4.15

Jul
Aug
Sep
Oct
Nov
Dec

4.14

3.70

3.07

1.90

1.26

0.66

You may have issues with "panel tilt" outside of the April through August boating season (if you mount the panels "flat" to the roof of the boat.

Last question--Are you doing this to save fuel costs or reduce smoke and noise? Especially in winter, it may be difficult to justify solar (of course, winter boating may not even be an option due to ice?).

-Bill

BB. · September 2014

Re: Battery questions for trolling boat conversion

Regarding the generator--It does depend on how long you want to run the genset too...

You can choose to charge from 5% to 20% (or even 25%) rate of charge. If you can run the genset for a number of hours--I would probably look at 5-10% rate of charge with a little inverter-generator (Honda eu2000i or Yamaha--230 VAC Euro voltage/frequency equivalent model).

200 AH * 59 volts charging * 0.05 rate of charge = 590 Watt charging power

590 Watt charger * 1/0.80 charger eff * 1/0.80 generator derating = 922 Watt (VA) rated genset

To go from 50% to 80% state of charge at a 5% rate of charge would take around 6 hours of generator run-time.

If you go 20% rate of charge--You are at a 4x larger genset or near 2kWatt rated. You can charge quicker (1.5 hours will get you from 50% to 80% state of charge). However--If you are trying to charge to >90% state of charge, then you need 2-4+ hours of "absorb" time (where battery charging current gradually reduces from max charger current to ~2-1% of battery AH capacity).

This last part of the charging cycle does not need a large genset--And can be very fuel inefficient (running a 2kWatt genset at 500 watts or less to finish the charging cycle to near 100% state of charge).

Is that old 10hp Diesel looking better yet?

-Bill

mike95490 · September 2014

Re: Battery questions for trolling boat conversion

sadly, if the diesel is still running, keep it. Not as sexy as solar/electric, but you know it gets the job done. Even check out the cost to repair it, may be less then battery/solar conversion.

My prime backup generator is a 1920's style lister diesel, and they were built like tanks. The electric start generator is for the un-sophisticated users in my absence.

samppa · September 2014

Re: Battery questions for trolling boat conversion

I know, costwise makes no sense to convert.. but i would like to do it

Now I wonder if it would be possible to install an electric motor parallel with the old diesel?
So that u can use electric to turn the prop, and when batts are low, diesel would be started. Then electric motor would freewheel. Could maybe charge batteries from the diesel at the same time.
Hybrid boat

I found that this kind of boat is (maybe) called displacement craft

AuricTech · September 2014

Re: Battery questions for trolling boat conversion

Just out of curiosity, have you looked at any of Torqeedo's products?

ETA: Added link to Torqeedo's US page, along with this link to Torqeedo's German-language home page.

BB. · September 2014

Re: Battery questions for trolling boat conversion

A "displacement hull" is just a relatively slow speed boat (typical sail boat, tankers, cargo ships, etc.) that does not go fast enough to "plane" on the water surface (like a speed/ski boat with a planning hull).

Does your present motor have a shaft disconnect (clutch, transmission, etc.) to free run the boat shaft? In some cases, transmissions will not properly move oil around with the engine shut down and a turning prop/tail shaft (not sure about boats, but this is a common issue with towing cars behind other cars/trucks/recreational vehicles--The transmission fails because it does not lubricate properly when the vehicle is towed and the engine is not running).

The Torqeedo or similar "outboard style" that Aurictech suggested is probably a better idea than trying to share the one shaft with both diesel and electric motor. If you cannot mount on the rear of the boat (not a flat transom), I have seen ones that simply use a structure and hang the motor to the side of the hull or you could build a transom box (or whatever they call it) inside the boat to mount the motor.

Adding a chain or belt to the DC motor off the side of the shaft may cause bearing problems--And you probably don't want the electric motor turning when the Diesel is running either--So another clutch issue.

If you have a separate motor+propeller for the electric motor, you could get a folding propeller for the Diesel engine to reduce drag.

-Bill

Battery questions for trolling boat conversion

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