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Need help with PV to power an air compressor - Page 3

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Posted by clare on May 23, 2015, 2:24 am
On Fri, 22 May 2015 17:19:05 -0400, "Jim Wilkins"

A different story with a bank of 8 batteries to charge on a daily
basis. I had a rather expensive "intelligent" 4 mode charger.
Didn't help when over the winter the power got disconnected, the
batteries got low, and they froze. AGMs are much more tollerant that

For normal SLA duty I've gotten over 12 years service out of flooded
batteries too, in daily use.

And how old were the "second hand" vlra batteries?

Equalizing charge on starved electrolyte batteries doesn't accomplish
much, so is not recommended.

Posted by Jim Wilkins on May 23, 2015, 12:16 pm

I agree that AGMs are less fussy. When I was the battery tech at  
Segway I learned how to charge and maintain new-tech sample batteries,  
mainly various types of Lithium, with voltage and current controlled  
lab power supplies. At another contract job I had some medical  
instrument lead-acids to repair, and the battery makers' data sheets  
showed that the same methods would work well. The procedure is simply  
to follow the data sheet's recommended voltages and currents.

Automatic chargers are faster, of course. Manual charging allows  
higher voltage at limited current to prolong the life of old  

I've built my own metered power supplies for this and similar tasks  
like reforming old electrolytic caps, and haven't looked very hard at  
commercial "intelligent" chargers with equalizing and desulfating  
functions. My quick impression of them is that they employ an  
open-loop brute-force method since they can't track electrolyte  
specific gravity to know when to stop. Also they could destroy  
attached equipment if they applied an effective voltage to a bad  
battery that was still connected to it. This task can be dangerous and  
requires some care and knowledge.

One of the Segway engineers left his very sophisticated charger for RC  
airplane LIPO batteries in the lab. Other than balancing multicell  
packs it did the same things we could with the lab supplies, but  
faster since it could monitor voltage and terminate a fast charge. It  
wasn't any better than a lab power supply when we needed to set the  
top-off voltage and current limits from a battery supplier's data  


The handwritten dates on the PowerSonic 12V 18A VRLAs are from 2009 to  
2011. I think that's when they were checked after scheduled removal  
from service.


Posted by adsDUMP on May 16, 2015, 7:43 pm
 You need several hundred watts of solar panels to keep the compressor
running even a few minutes.  You probably need about 800AH of
batteries to get 3 hours of compressor use..  You need to know the
starting current of the compressor - which is typically several times
the running current - and the inverter must be big enough to handle
that load (consider 5000 watts minimum).  Your wiring should probably
be 1 gauge or heavier.

See this page to calculate how long you could run the compressor on
your batteries when fully charges: http://dx9s.net/5-2/battcalc/  
With 3 35AH batteries, you *might* get 20 minutes of compressor RUN
time - if you had some way of getting it started.

The HF 45 watt panel set delivers less than 4 amps in full sun.  That
woulod require more than 9 hours of full sun to recharge EACH of your
35AH batteries.  

From where you currently are with solar power, it would be cheaper to
just buy a generator...

Posted by New Guy on May 16, 2015, 8:04 pm
 On Sat, 16 May 2015 15:43:52 -0400, adsDUMP wrote:

  Thanks for your input. I guess I've taken the HF setup as far as
 I need to. I'll check out the link you posted and start researching
 better panels/accessories.

Posted by mike on May 16, 2015, 8:32 pm
 On 5/16/2015 9:21 AM, New Guy wrote:

I missed the part where you disclosed where you live and how long
per day you need to run the air compressor.

Go here:
and put in your location.

This program is obsolete and you can't change anything.  Just click thru
to the results and scale the numbers by 45/4000 to get numbers for your
45W panel at your location.

Yes, there's an upgraded site, but you'd have to figger out how to work
it and gain nothing.

Notice the pitiful amount of energy you'll get, especially in winter.
Your batteries have to supply enough energy per day to run your
air compressor.  And you can't run 'em flat, so double it.

Your panels have to produce enough energy to recharge the batteries
every day in winter months.  That gives you zero allowance for
cloudy days, so multiply the number by whatever makes you comfortable.

Get the resultant numbers.
When you recover from the shock of the magnitude of the problem
in size and $$$,
Go buy a gas generator.  Or a long extension cord.

The same calculations work for lighting.  If you really don't need the
air compressor much, you may be able to do a small amount of lighting
with your current setup.  Run the generator only for the compressor.
Or put a gas engine on the compressor.

Solar makes a lot of sense when you have low requirements and absolutely
no way to get power any other way.

Stated another way, solar is rarely cost effective.

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