Is there really a current below which batteries won't charge?

[shmoo]

Is there really a current below which batteries won\'t charge?

In recent discussions about solar panels several posters seemed to be suggesting that there was a lower input current limit below which batteries could be expected have their charge maintained, but wouldn't actually increase their charge.

If this is true it has implications for e.g. mounting of solar panels - on a swinging mooring on an east-west stretch of river it would be better to put both on same side to get above the lower limit and actually do some charging, even if only once a tide. (Rather than one on each side to catch sun on ebb and flood, but risk not charging at all becuase current too low)

Also, what is the lower limit. C/100? C/200? C/500? Any links or references to battery manufactures figures on this.

 

[lw395]

Re: Is there really a current below which batteries won\'t charge?

Not aware of any threshold current that needs to be exceeded, but batteries do tend to have a self discharge current, which you need to beat on average. This varies alot with the state, age, brand etc of battery, but for instance, I have an old 55AH ups battery in the garage drawing only about 10mA at float voltage. Some dodgy motorbike batteries I have draw nearer 100mA even after being floated for a long time.
I think I would aim for max integrated current over the day, with a bias towards max charging hours, but would be interested in others experience!

 

[William_H]

Re: Is there really a current below which batteries won\'t charge?

A car battery could be expected to self discharge in perhaps 6 months of idleness. This period is probably indicative regardless of size of battery. So 100AH battery might lose 60 AH in 6 months so 10AHper month .3 AH per day.
A 10 watt solar panel might give .5 amp for 6 hrs in summer or 3 AH per day. This depends on panel orientation. Swinging mooring is less.

But while these figures might be wildly inaccurate you can see that a well oriented 1 watt panel might just keep a 100AH battery charged.
Obviously 5 watt panel would be a lot safer and 10 watt would give you some recovery. For smaller batteries current requirement is proportionally less.

Regarding panels on swing moorings I have 5 watts on a swing mooring which I strap to the boom cover at an angle to account for the common wind direction. In summer tides have less effect than in UK. This will recharge a 14AH NiCad wet battery over a few weeks. But then we get a lot of sunshine.
There is no answer for your orientation question if you are on a swing mooring. If the boat has any predominance of direction then use that other wise horizontal is as good as any.
I don't think Iw395's current reading that a trickle charger put in are a real indication of actual current loss of a battery by being stagnant. I think most of his current from the charger is being wasted.

good luck olewill