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Pv/Bat Ratio

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Brian Graham

Jan 1, 1970
0
A friend was talking to an _expert_ on batteries yesterday. The understanding that I have based on his conversation - ever played telephone?? - is that the PV current should really be no less than 5% of the bank size. Ie, a 5 amp PV panel should be connected to no more than a 100ah battery bank. I believe this has something to do with internal battery resistance and the inability of lesser pv currents to actually charge the bank.

This means that a 150 Watt 5A pv panel, which in 7 hrs of sunshine could pull in 35ah daily, is unable to charge a battery larger than 100ah. Actually there's _some_ wiggle room so call it 200ah. ???????

Here's something else that comes to mind. The panel above is a 24V panel. So, it sounds like it could successfully charge four 6V 100ah cells connected in series (400ah total), but could not charge a single 24V 200ah bat (if there was one). Most Odd...

This bodes well for my 15Watt panel at my trailer 'charging' my 100ah battery.. :-(

I need some help to get a good handle on this.
 
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William P.N. Smith

Jan 1, 1970
0
Brian Graham said:
A friend was talking to an _expert_ on batteries yesterday. The understanding that I have based on his conversation - ever played telephone?? - is that the PV current should really be no less than 5% of the bank size.

The other way to look at this is the the peak charge current should be
no less than C/20.

I wouldn't call this a hard rule, but it's not bad guidance. Charge
currents that are too low will prevent the battery from ever achieving
full charge, which will drastically shorten it's lifespan.

OTOH, I'm charging a pair of 225AH golf cart batteries with 40W of
solar panel, so I have some evidence that C/75-ish rates aren't
immediately fatal. Note that you still need to stay within your power
budget...
 
H

Hawker

Jan 1, 1970
0
Most of my systems are for emergency back up for Radio communications.
As such I usually have way more battery than panel.
My systems tend to range from 100ah to 600ah of battery and 32W to 250W
of panel.

What I have found, like this poster is the magic number seems to be
around C/75 or so. Any less and I can't get the battery to charge
properly. However C/50 seems to do better than C/75. Above C/50 I don't
see much improvement. Conversely, If I charge above C/16 I start to get
degraded battery life and the battery doesn't charge as well. I tend to
shoot for C/25 or so on my systems.

Hawker
 
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Brian Graham

Jan 1, 1970
0
What I was planning on was a small 70W? panel to power 200W CF lights and ceiling fans upstairs. Non-critical loads. I would have enough daylight in the summer to power the load; in the winter I would have to recharge the bats on the weekend. I was also planning on a _substantial_ battery bank. That way, when power is out, I have a reserve for my fridge/freezer.

Clearly, I plan on gen/mains recharging in the winter. Do I have to plan on the same in the summer due to current limitations? The daily ah would be sized to suit the consumption, but it sounds like it would be insufficient to recharge the bank properly.
--
Brian


And that's another essential issue. If you have a fixed amount to spend
on the system, but intend to expand it substantially over time, my
suggestion would be to tilt it towards batteries rather than PV, and
count on using a generator powered charger occasionally to top up the
batteries. The PV can serve as trickle charge and maintenance at first.

My thinking on this is that it's better to have a consolidated large
battery bank and add PV as funds become available, rather than adding
batteries. The batteries are much more age-sensitive...

DJ
 
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Brian Graham

Jan 1, 1970
0
Yes, the 200 watts is a cumulative total of my cf lights and bedroom ceiling fans at home.

The thread was about trying to understand what is needed for panels with respect to charging a battery bank.

I did cite my intent on a small 70W panel, the need to recharge the bats in the winter due to the short daytime (2.5) hours. In the summer, I would have enough hours to replenish my usage. BUT, I was wanting a substantially large battery bank, not sized for above, but rather sized to run a fridge/freezer for a few days.

It was suggested that a 5amp panel (24V, 120W) panel wouldn't properly charge a battery bank much larger than 100ah due to internal battery resistance. My intent would have been more along the lines of 400-800 ah bank. Panels sized for actual usage, but a battery bank big enough for emergency usage.

Yes, I made a comment questioning the ability of my 15 watt panel to properly charge the 100ah battery I have in my travel trailer. Obviously if a 5 amp panel is required to charge the battery properly, my 1 amp panel won't.

So, in the home with a 70 watt panel, 200W x 2 hr load, and a 800 ah battery bank:

1) Obviously I will need to recharge the batteries in the winter, on the weekends.
2) In the summer, enough power will be captured to run the load. Will I need to recharge from a gen/mains anyway to properly charge the bank?

3) At my trailer, with a 15W panel and a 100ah battery, and only 2 ah/day loads on the weekends, will I need to recharge the battery from the grid since the panel current is so low? (Same problem as point 2, really..)

Hopefully I've been clear. I've only been considering requirements based on ah captured and ah used, and haven't an understanding of pv sizing for battery maintenance. I've always assumed that a 25% excess would provide a recharge 4 days after utilization, without consideration given for the battery resistance.

I've always figured if I had a 50 ah usage per day, and you wanted 4 days autonomy and a 50% DOD you would want a 400ah bank. And an 800ah bank would double the autonomy. And so on. Never considering that a small panel which is big enough to meet the 50 amp usage, might be insufficient in size to recharge an 800ah bank, for example..

Any clarification/confirmation welcomed.
--
Brian

Brian said:
What I was planning on was a small 70W? panel to power 200W CF lights and ceiling > fans upstairs.

Have you considered a DC ceiling fan?
And I assume 200w CF lights is a typo? or is that a cumulative number,
ie, 20 x 10w?
Non-critical loads. I would have enough daylight in the summer to power the load; in
the winter I would have to recharge the bats on the weekend. I was also planning on a >_substantial_ battery bank. That way, when power is out, I have a reserve for my
fridge/freezer.

Ok, you started the thread on a 15 watt panel and a trolling motor
battery in a trailer, and now we're talking about fridges and ceiling
fans and 200 watts of lighting for a home-style system.

Might you give us a quick overview of what you want to do?

DJ
 
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