SmartSolar 75/15: fuse to battery blown twice

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lemon asked Jan 16, '24

SmartSolar 75/15: fuse to battery blown twice

Hi everybody,

I have a 280W peak panel on my camper, worked with the SmartSolar 75/15 flawlessly for 3 years now. A few weeks ago the SmartSolar suddenly did not charge the battery anymore and gave strange battery voltages (p.e. 7V when the battery was at 13.4V).

Last week I had time to check and noticed that the 15A-fuse in the 20cm long 10AWG cable to the plus pole of the battery was blown. So I changed cable and fuse and the SmartSolar charged the battery again. Good.

Two days later the fuse was blown again. Strange. we have winter here in Italy, a 280W panel can't produce as much current as needed to blow the fuse away, and then the SmartSolar is limited and should not send more than 15A to the battery.

My reseller told me to use a 20A fuse on the cable to the battery, but I can't understand why it happens now and not in summer when we have much more current coming from the panel.

What do you say?

Thank you very much!

MPPT SmartSolar

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Answer 1 · 2024-01-16T10:40:31Z

Paul B answered · Jan 16 at 10:40 AM

I would do as you have been requested. you may find its just surge that causing it, anyway a 20 amp fuse will tell the story

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Answer 2 · 2024-01-16T10:42:11Z

klim8skeptic answered · Jan 16 at 10:42 AM

@lemon What do you say?

You sure can blow a fuse if it gets hot due to a poor holder or cabling.

Check your mppt's history to see what the P max has been.

Check your wiring and fuse holder are ok for a 20a fuse, and fit one.

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Answer 3 · 2024-01-16T14:16:44Z

lemon answered · Jan 16 at 02:16 PM

Hi,

here you can see the values just before the second fuse has died, maybe this can help to understand. But for me these values are ok, they look quite typical.

2024-01-16-15-13-44-t5solar-vrm-portal.png (24.3 KiB)

2024-01-16-15-17-27-t5solar-vrm-portal.png (23.9 KiB)

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klim8skeptic ♦ commented · Jan 16 at 07:44 PM

@lemon Looks like you setup can get to 15a even in winter.

Do you have any loads on the load output?

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ejrossouw klim8skeptic ♦ commented · Jan 17 at 07:51 AM

Current is very low around 2A so are you confusing V with A in graph?

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klim8skeptic ♦ ejrossouw commented · Jan 17 at 09:41 AM

@ejrossouw Battery current is above 14.25a @ noon.

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ejrossouw klim8skeptic ♦ commented · Jan 17 at 07:30 PM

@klim8skeptic 14.25V not A. Current scale is depicted in orange on the right and shows just over 4A ;)

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klim8skeptic ♦ ejrossouw commented · Jan 17 at 08:01 PM

@ejrossouw your right :)

Wonder what the history looks like.

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solar-history.png (161.1 KiB)

lemon klim8skeptic ♦ commented · Jan 18 at 09:37 AM

This was from the day the fuse was blown:

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2024-01-18-10-36-08-t5solar-vrm-portal.png (16.5 KiB)

klim8skeptic ♦ lemon commented · Jan 18 at 09:52 AM

@lemon Cool, doesn't look like that would pop a fuse. Unless the wire has been pinched / rubbed through.

Do you have any thing connected to the load output terminals? An overload there will pop the batt fuse.

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lemon klim8skeptic ♦ commented · Jan 18 at 10:29 AM

No, loads are connected directly to the battery (through another fuse box obviously).

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Answer 4 · 2024-01-16T14:43:07Z

pim57 answered · Jan 16 at 02:43 PM

Maybe one of the cables (pv or battery) wore out in these three years and touches the frame somewhere?

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Answer 5 · 2024-01-17T06:48:42Z

kevgermany answered · Jan 17 at 06:48 AM

The automotive inline fuses are usually rated at a continuous current of half the max. You may need to go even higher than 20A. But as said, check the cabling.

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Answer 6 · 2024-01-17T19:45:33Z

ejrossouw answered · Jan 17 at 07:45 PM

@lemon My money is on a bad connection or cable in light of it having worked for 3 years without problem. Vehicles and movement, normal oxidation heat etc all impact connections and even cables over this time. Based on my own experience with numerous customers who experienced erratic voltages, it has always been the place to start.

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Answer 7 · 2024-01-18T00:22:44Z

k-k answered · Jan 18 at 12:22 AM

What type of batterie do you have, lead acid or LiFePO4?

What temperatures do you have (had on a windy hill -15°C)?

Enough water in LA batterie or heating on/off for LFP?

What is switched shortly before 14:00 first picture shows heavy load (low voltage but 0A to see*), second picture shows no load (0A but high voltage) a that time -> 'polar batterie' with rotten heat mat?

*maybe frozen elco inside MPPT

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lemon commented · Jan 18 at 09:39 AM

Hi,

battery is LiFePo4 without heating mat, temperatures in the last weeks were never < 5°C in the car.

The only load I have in the car (besides the Raspberry with Venus OS) is my freezer which switches on a few times a day consuming about 5A for some minutes only).

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