Charging Trolling Motor Batteries

[Ketchikan]

I am designing a small electric boat that will use the following;

1 - 36v trolling motor with a draw of 42 amps per hour

3 - 12v batteries 115 ah x 3 = 345 ah

All components will be within 3 feet of each other.

I have not setteled in on panels to use yet but I will use the Flexmax 60 MPPT CC

Assuming just one hour of use drawing 42 amps from the batteries given

A. 3 - 12v 140w 8 amp solar panels = 420w 24 amps

B. 3 - 24v 240w 8 amp solar panels = 720w 24 amps

Here is where I get confused...

Do Amp hours from one panel equal Amp hours from another panel?

In one hour of perfect sunlight will I only recharge my batteries with A. by 24 amps and with B. by 24 amps meaning that there is really no difference on the size of panels I use as long as I get 24 amps

or

am i I missing something?

Thanks for your answers

[inetdog]

I have not setteled in on panels to use yet but I will use the Flexmax 60 MPPT CC

Assuming just one hour of use drawing 42 amps from the batteries given

A. 3 - 12v 140w 8 amp solar panels = 420w 24 amps

B. 3 - 24v 240w 8 amp solar panels = 720w 24 amps

Here is where I get confused...

Do Amp hours from one panel equal Amp hours from another panel?

In one hour of perfect sunlight will I only recharge my batteries with A. by 24 amps and with B. by 24 amps meaning that there is really no difference on the size of panels I use as long as I get 24 amps

or

am i I missing something?

Thanks for your answers

Yes, you are missing quite a lot.

The nominal 24 volt panels will produce twice the voltage that the nominal 12 volt panels will produce. But if you had chosen a PWM charge controller, you would only get the same amperage from either A or B.

What you are missing though, is that you cannot both multiply the voltage of the panels by 3 to get a high enough voltage for your 36 volt battery bank (putting the panles in series) and at the same time multiply the current by 3 to get 24 amps.

Both A and B will produce 8 amps. A will produce 8 amps at 45 volts. B will produce 8 amps at 90 volts.

But since B will give you twice the power, and twice the voltage you need, an MPPT charge controller will convert the ~90 volts at 8 amps to ~ 45 volts at 16 amps to send to your batteries. Even then, you are looking at 3 hours of full maximum power from array B to put most of the 42 Amp Hours back into your battery bank. However, the last 10-20% of the recharge will be much slower as the batteries move into the Absorb and Float stage of charging and the full power from the panels will not be usable.
It would take you two days to recharge completely for every hour you run the trolling motor at full power by your calculation.
However, there is a more serious problem even than that:
Your 3 12 volt batteries at 115AH each will be wired together in series to produce a 36 volt battery, still at only 115AH.
So your 1 hour of use will be taking close to 50% of the energy from a fully charged battery bank. (At the discharge rate of 42 amps, you will not get the full 115 AH.)
That will kill your batteries in a relatively short time (~100 days of use, maybe.)

[Ketchikan]

Thanks, I knew better about the 3 * 12v batteries in series equal the same 115 ah.

But you did remind me about not reducing my batteries below 50% .

I was only using the one hour as a relative fiqure I will have at least 8 to 9 hours per day and would only use the motor on full approximately 2 hours for a total of 84 amps drawn.

The panels would be charging while I was using the motor so I know everything is not equal but in a perfect situation I would be drawing 42 amps and replacing it with 24 amps leaving a net of 18 amp drawn of 15% on the batteries or 97 amps left for the first hour and 36 amps drawn of 31% on the batteries or 79 amps left for the second hour and the batteries would be fully charged by the end of the day.

Since I can't calculate all of the lost power from the panels to the batteries I would think that I might not destroy my batteries as quickly as it seems.

[Ketchikan]

OOPS I misread your ecplanation that the MPPT will convert the 90 ~ 8 amps to 45 ~ 16 amps so when you say I have 3 hours with the B setup is that it will take 36 hours of sunlight just to put back in 42 amps?

[inetdog]

OOPS I misread your ecplanation that the MPPT will convert the 90 ~ 8 amps to 45 ~ 16 amps so when you say I have 3 hours with the B setup is that it will take 36 hours of sunlight just to put back in 42 amps?

No. I did not say three days, I said three hours. But with the way the charger actually works as the batteries get above about 80%, it will actually take more like 6. And "full sunlight" for the panels will probably be at most 5 hours per day during the summer. As little as 1 or 2 in the winter.

When the sun angle is low in the sky, you are not getting anything close to full output from your panels.

While you are using the boat, will you be able to keep the panels aimed at the sun? And will they be free of shading from trees, etc?

[Ketchikan]

Thanks again.

The panels would be free from shading and I would be able to come close to the correct angle where I will be about 70% of the time.

[Ketchikan]

If I was able to get 3 * 290w, 35.68 vmpp, 44.90 voc, 8.15 impp panels could I get 24 A per hour output with the MPPT?

[Sunking]

If I was able to get 3 * 290w, 35.68 vmpp, 44.90 voc, 8.15 impp panels could I get 24 A per hour output with the MPPT?

You got me confused. Is the motor 12, 24, or 36 volts?

With 870 watt panels running at Vmp of 40 volts on paper will deliver roughly 36 amps to a 24 volt battery assuming it is in a discharged state.

[Ketchikan]

Sorry for the confusion but the motor is 36v and draws 42 amps per hour at high speed.

[Naptown]

for perhaps a couple of hours at most during the summer
what is the insolation where the intended use is to occur?

[Ketchikan]

Insolation is approximately 5 during 3 peak summer months

[Sunking]

Sorry for the confusion but the motor is 36v and draws 42 amps per hour at high speed.

OK so why are you asking about 24 volts? This is not going to work out like you think it will.

If you were to run the motor at high power for 1 hours will use roughly 1500 watt hours. So if you want to replace that and lucky enough to get say a 5 Sun Hour day would take a 500 watt panel a whole day to replace the power. Take that same battery down to 50% DOD and it would take a 800 watt panel all day from sun up to sun rise to fully recharge.

[Ketchikan]

Not 24v a 36v motor drawing from a 36v battery bank.

[Ketchikan]

I am not sure where 24v came into the picture but you could have gotten it from my post of "If I was able to get 3 * 290w, 35.68 vmpp, 44.90 voc, 8.15 impp panels could I get 24 A per hour output with the MPPT?" where i should have said 24amps per hour.

Could you please explain where I use 1500 watts?

Thank you.

[Ketchikan]

Okay volts * amps = watts or 36 * 42 = 1500