First post. Battery longevity question

[hominid999]

Hey folks, glad to be here. So my question to pose to the collective wisdom in this forum is: how long should a marine battery run a 15 watt appliance through a 300 watt sine wave inverter? It's charged via a 7 amp Coleman charge controller thanks to two 40 watt solar panels. The charge controller doesn't have a float charge; it just cuts out when the green LED says "Charged", and starts charging again (according to the manufacturer) when it hits 13 volts.

I've come to find out that converting amp-hour to cranking-amp battery ratings is quite the research rabbit hole, so it's hard to use straight math to figure out what to expect from my battery. It's at least 5 years old, and I've always kept it charged up with a 500 MA trickle charger. Which comes to my second point: The charge controller doesn't top the charge up like the trickle charger, so I get a bit more time out of it, but that won't help me when I'm out camping... My solar rig is new, and I'll get a new battery if needs be.

Thanks in advance.

[Mike90250]

Make & model of the 300w inverter. unless it's a good one, it's internal losses / consumption will be more than your 15W load


Most marine batteries do have a 20hour rate AH rating. There is no way to convert CCA to AH.

A generic $95 group 24 marine is 12V @ 90ah (group size 27, another 20ah more or so) for a total of 1080 watt hours. The guideline is to only withdraw about 20 or 30% of the capacity on a daily basis, and strive never to go below 50%, which starts to drastically reduce battery life.

15w for the appliance and another 30w for inverter losses, and you are looking at 45watts consumed

20% 864wh remaining = 4.8 hrs
30% 756wh remaining = 7.2 hrs
50% 540wh remaining = 12 hrs

But only holds true if the battery is well charged. The other thing to look at is your recharge watt hours.

80W of panels might give you 80% of nameplate, when well aimed. Now you have only 64W of PV for 4 usable solar hours is only 256wh replacement. You finish the math...

Your batteries are in a death spiral

[hominid999]

Make & model of the 300w inverter. unless it's a good one, it's internal losses / consumption will be more than your 15W load


----snip----

Thanks for the response Mike... So, my inverter is a Bestek MRZ3011HU, show here on Amazon: https://www.amazon.ca/BESTEK-Power-I.../dp/B01DP9JE18

A couple reviewers did some detailed testing and found it to only draw between .5 and .7 amps when idle, so it seems fairly efficient. For my marine battery, at what voltage would it be considered fully discharged? You say below 50% drastically reduces battery life - I thought the point of a marine battery is that it *can* be deep cycled?

[Mike90250]

Automotive batteries can only withstand 5-10% discharge

Deep cycle batteries are rated 10-30%, deeper than 50% accelerates degradation.

0.7A @ 12V = 8.5 watts, better than I thought, so you will get quite a bit more life out of the battery

Morningstar Suresine 300
- the SureSine consumes 450mA
while powering loads. During no load conditions, solar energy is
not wasted because the SureSine automatically powers down to
stand-by mode, reducing self-consumption to one tenth of
operating consumption.
https://2n1s7w3qw84d2ysnx3ia2bct-wpe...NG_R2_1_08.pdf

Battery-DoD-vs-Cycles.gif


battery-state-of-charge LeadAcid.jpg

[hominid999]

The saga continues, in that I discovered my battery had a bad cell. Now that it's replaced, I also upgraded my charge controller to one that maintains a float charge. Yeah me! The LCD display on it shows the solar panel amps (I have two 40 watt panels), and the highest I've seen it reach is 3 amps on a clear day, with them pointed accurately. The specs for each panel says 2.3 amp output, which doubled is 4.6. 65% efficiency seems odd to me.

The ambient temp is about 27 C (81 F). My cable run is about 16 or so feet, but do not know the gauge. Input, questions, and advice welcomed.

[MichaelK!]

The saga continues, in that I discovered my battery had a bad cell. Now that it's replaced, I also upgraded my charge controller to one that maintains a float charge. Yeah me! The LCD display on it shows the solar panel amps (I have two 40 watt panels), and the highest I've seen it reach is 3 amps on a clear day, with them pointed accurately.

Most likely the reason for the bad cell is the fact that you're chronicly undercharging the battery.

For doing the math, let's assume your battery is 100ah. The math for optimal charging would be
Xah x 13% x 14.8V charging voltage x conversion efficiency = panel wattage needed.

So, that works out to be.....

100ah x 0.13 x 14.8V x 1.25CE ~240watts

Now you know why you had a failed battery. You've been attempting to do with 80watts what you should have been doing with 240watts. Keep in mind though that those numbers are with a good MPPT controller, not the cheapo PWM controller you have. Using your own numbers, your panels could theoretically produce 80Watts/14.4V =5.555amps, but you're getting 3.0, so your CE factor is closer to 1.85.

If you got a 250W grid-tie panel producing about 30V, then connected that to an MPPT controller to transform the voltage to 14.8, you'd have a well performing system.

[hominid999]

Most likely the reason for the bad cell is the fact that you're chronicly undercharging the battery.

For doing the math, let's assume your battery is 100ah. The math for optimal charging would be
Xah x 13% x 14.8V charging voltage x conversion efficiency = panel wattage needed.

So, that works out to be.....

100ah x 0.13 x 14.8V x 1.25CE ~240watts

Now you know why you had a failed battery. You've been attempting to do with 80watts what you should have been doing with 240watts. Keep in mind though that those numbers are with a good MPPT controller, not the cheapo PWM controller you have. Using your own numbers, your panels could theoretically produce 80Watts/14.4V =5.555amps, but you're getting 3.0, so your CE factor is closer to 1.85.

If you got a 250W grid-tie panel producing about 30V, then connected that to an MPPT controller to transform the voltage to 14.8, you'd have a well performing system.

Thanks for the interesting reply Mike! Very educational. So, a couple things:

Firstly, most of that battery's life was spent on a float charger, then used occasionally to power my UPS when the power went out. It's just in the last few months I've been playing around with this little solar rig, and float charging it to prove my old charge controller was useless, in that it would maybe charge it to 75% capacity. Hence the new staged charging controller that maintains a float charge. I just got my new battery maybe a week ago, and it hasn't been all that sunny of late, so I'm still testing.

The panel manufacturer specs state the output to be 17.5 volts, though I'm sure that drops when cloudy. So your formula may need tweaking?

Thoughts?

[Sunking]

Most likely the reason for the bad cell is the fact that you're chronicly undercharging the battery.

For doing the math, let's assume your battery is 100ah. The math for optimal charging would be
Xah x 13% x 14.8V charging voltage x conversion efficiency = panel wattage needed.

So, that works out to be.....

100ah x 0.13 x 14.8V x 1.25CE ~240watts

Now you know why you had a failed battery. You've been attempting to do with 80watts what you should have been doing with 240watts. Keep in mind though that those numbers are with a good MPPT controller, not the cheapo PWM controller you have. Using your own numbers, your panels could theoretically produce 80Watts/14.4V =5.555amps, but you're getting 3.0, so your CE factor is closer to 1.85.

If you got a 250W grid-tie panel producing about 30V, then connected that to an MPPT controller to transform the voltage to 14.8, you'd have a well performing system.

Micheal that is not the way it works. To start a 12 volt battery with 14.8 volts is fully charged and would take almost 0 amps. 2nd 13% or C/8 charge rate would be max charge rate which may or may not be needed. Most likely not unless you live in a place like the pacific northwest. With MPPT controller and a 100 AH battery 120 to 150 watts is more then enough.At 240 watts on a discharged battery would produce 19 to 20 amps of charge current or C/5 which is too much.