Bit of help needed with my maths

[Bonnox]

Hi guys just needing a bit of help, I'm in the process of building my off grid shed, I'm going to be using PVC panels and deep cycle batteries but currently can't afford the control unit to switch off the solar charging and not over charge the battery, and probably won't be able to till after Christmas now, so was going to use the two batteries I have and use one while one is charging and swap them around, just need the help with the maths to make sure I don't over charge them, and I'm not massively up on my technology and current conversion etc so.....

If I use for example a 75Ah battery I can use something that uses up 1 amp per hour for 75 hours,
And if the solar panel puts out/charges at 2Ah then it would take 37.5 hours to charge from flat till full, am I right in this ?

Many thanks for your guys help

[Lightspeed]

Hi Bonnox

Your math is correct.

Your panel is rated at 2A ( 24w at 12v 48w at 24v) This will be peak output at optimum operating temperature. Conditions to achieve peak output are cool and clear midsummer days with sun overhead. Output drops off with greyer winter conditions, and of course the number of daylight hours drops off at that time of year too.

A charge controller for your setup need not be expensive. This one will do the job. It's rated 10A and is 12v and 24v compatible, price is less than £7 delivered: http://www.ebay.co.uk/itm/10A-CMP-Solar-...5af59c51ea

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BTW although your 75 AH battery is able to deliver 1A for 75 hours, it is good practice never to completely drain the batteries as this will shorten their operating life.

Aim for 30% discharge before recharging to prolong their operating life.

[Bonnox]

(10 October 2013, 06:13)Lightspeed Wrote: Hi Bonnox

Your math is correct.

Your panel is rated at 2A ( 24w at 12v 48w at 24v) This will be peak output at optimum operating temperature. Conditions to achieve peak output are cool and clear midsummer days with sun overhead. Output drops off with greyer winter conditions, and of course the number of daylight hours drops off at that time of year too.

A charge controller for your setup need not be expensive. This one will do the job. It's rated 10A and is 12v and 24v compatible, price is less than £7 delivered: http://www.ebay.co.uk/itm/10A-CMP-Solar-...5af59c51ea

---

BTW although your 75 AH battery is able to deliver 1A for 75 hours, it is good practice never to completely drain the batteries as this will shorten their operating life.

Aim for 30% discharge before recharging to prolong their operating life.

Brilliant thanks, I had not seen that one the ones I had seen were coming in at about £50, guess I was looking at the really high end ones, thanks

[Nix]

Hi Bonnox, if I can just add to what Lightspeed has already said, do NOT flatten your batteries! I tend to go for 20 -30% discharge as a good compromise between battery work and life expectancy.

What is always quoted for an Ah rating is in ideal conditions, you'll rarely see this, not only with the sunshine, temperature and duration of day but also in the condition of the battery itself. Once you discharge a battery completely, you effectively damage it's ability to fully recharge. If your batteries are already used ones then the ampage you'll get from them will be much less than quoted. The type of battery is important too. It's good you are using deep cycle batteries, if it's a normal car battery then they're built for quick high amp duty for short bursts eg starter motors but ultra deep cycle batteries are for long lasting slow discharge work. The deeper you discharge (called DOD or "depth of discharge") the shorter the battery cycle life.

If your panels are only giving out 1 or 2 amps max then this is regarded as trickle charging really. You may be surprised at how long it will take to recharge your batteries with just a trickle charge in typical British weather. What panels do you actually have, what voltage/wattage are they rated at? Just wondering if it would be better to use them together than one at a time. What are you powering and for how long, ie just day or day and night?

Had a quick look at the ebay link for the controller. I'd personally go for a UK based company supplying a PWM controller. This one is from Hong Kong and customs can and will charge you extra for it making it not so cheap. PWM technology is very common these days and far more efficient than the basic type of controller. As Lightspeed says, for the sake of a tenner, you really ought to get a controller. Try this one, it's PWM, sent from the UK and is PWM for £9.99 delivered.

Hope this helps!

[Bonnox]

Yeah cheers guys I don't intend to ever let it run down to empty, I was just using the example to make the maths simpler to write up, Im led believe the same applies for charging batteries as well, that if they aren't fully topped up they have 'memory' so the full capacity of the battery can be lessened say if you only charge it to 70% each time then the battery thinks that 70% is actually full so 70% becomes the new 100% as it were! or is this only for the smaller batteries ?

[Nix]

Lead acid batteries, which includes the ultra deep cycle batteries, do not have a memory. I believe it's only the NiCd and NiMH rechargables that have this memory effect.

[Skvez]

As Nix says Lead Acid batteries do not have a (noticeable) memory effect.
As a battery approaches begin fully charged a larger percentage of the power you provide to charge it goes into heat.

If you have a 75Ah battery and discharge it to 2/3 full (50Ah) it can take noticeably longer than 25 hours of charging at 1A to fully charge it again.
Efficiency and losses.