Hello AGAIN,

Back for more. Anyway. still kind of torn between battery choices. Basically inefficient batteries can be had cheap and thus more money either spent of battery array and or solar panel array.

I still have to run the numbers more. TO see actual which way gets me the most power for my needs. More panel, lower efficiency batteries. Or high efficiency batteries with small solar array. Obviously like to get large array and high efficienc batteries, but always is a budget.

I have read the AGM batteries have high efficiency as low internal resistance. up to 98%.

I have also read that normal lead acid is maybe 80-85% efficient. I was wondering where the Surrette fell, as they seem to be a bit of a mix between the two.

Any idea of efficiency?

I was kind of leaning to lower efficiency batteries with larger solar array.

This way when the batteries die, there is an option of buying lower expense batteries and not killing the system. Other way around if the system is designed for high quality agm and replaced with low quality lead-acid, would hurt the system.

Anyway any thoughts on efficiency and my idea?

I think all batteries of a chemistry class, have the same characteristics,
Add <chemical name> to make them rugged, and you get self discharge (forklift)
Seal them, and >90%.
Flooded deep cycle, I believe they will all be very similar. 85% ?

I thought I read that the surrettes plates were actually a bit different as they were coated, kind of semi glass mat if I remember correctly.
SO while there are lead acid, there are not quite normal lead acid. But still not pure AGM so not super efficient.

Sealed forklift are over 90%???

Also think you are thinking of "Lead-Antimony plates"

There is current efficiency and power efficiency... Batteries are pretty good at current efficiency--that may be where your 98% AGM number comes from.

But, from what I understand, the typical AGM is around 90% efficient when power is measured (P=V*I). And the typical flooded cell battery is around 80% efficient.

With older PWM charge controllers--power efficiency really did not matter too much because they could not maximize the solar panel's power to battery transfer (via MPPT--Maximum Power Point Tracking)--but simply transfered AMPs from the solar panels to the batteries.

Now, with MPPT type controllers (and using Inverters for AC power)--power efficiency is very important too. The MPPT charge controllers maximize power transfer (not current) from the solar panels to the charging batteries. And inverters, are also constant power devices too...

Say you want 1,000 watts from an inverter on a 12 volt battery bank--if the battery is at 14.1 volts I=P/V=70.9 amps; and at 10.5 volts -> 95.2 amps... In both cases, the power involved is the same (neglecting inverter efficiency and battery losses from the higher current).

My guess regarding the difference between 80% of flooded cell and 90% of AGM efficiencies is probably the conversion of water into hydrogen/oxygen gas and the requirement to stir the batteries with equalization...

Which leads to another cost that can really hurt some people--the costs and difficulties of obtaining distilled water to keep the batteries full--which can be several gallons a month for a large battery bank (I don't know the exact amount of water required).

-Bill

So with that in mind higher voltage battery bank would that help efficiency?

From your statement. I would assume 98% of high current, is a larger loss then 98% of low current high voltage.

if this is your first system it is sometimes advisable to go the cheaper battery route because if something is amiss in the system it could take out your batteries or reduce their lifespan. after the cheaper batteries have aged properly (or prematurely as by that you'd know and fix the cause of this) then you might consider the better agms. you may not get th 98% you site as i believe it closer to 90% for agms and other batteries about 80% or so. this does vary and don't forget that batteries age (even good agms) and that must be factored in the general percentages.

Just ran the numbers. and economically. Not accounting maintenance is much in the favor of a larger array. It will be much cheaper and the actually hit in performance can be over come with the extra money saved and spent on larger array. And when batteries die, and replaced by agm's they will just maximize the system energy.

This will be a large system so can not buy to cheap of batteries. Thinking of going with the surrettes but the lower S-530 battery. Not their latest and greatest but good enough, and cheap enough.

Few other things I am doing as well. Having a dedicated solar panel system for well pump. Will just do irrigation. Also thinking about doing the same for pool pump.

With these two things, first off dc will save a bit on inverter efficiency, but should also save some on battery efficiency, but also not having to have batteries I can scale these two things out of monthly usage. In my calculations they were about 100kwh of uses so that out of the equation will help alot.

Should make my main system a little more capable.

Regarding higher voltage (and lower current) equals higher efficiency...

In general, yes... Although there are specific times when running a 48vdc may not make sense...

1. Wind Turbine generator--many turbine have very poor output at 48 VDC vs 12 VDC.
2. Small system--12 volt inverters / chargers may cost less.

Regarding wiring--wasted power goes P=I^2 * R (I squared times R)... So, if you double the current, wasted power goes up by 4x. Cut the current in half, the power loss is reduced to 1/4...

Batteries... Look at the brand/model of batteries you want to use. You will find that a battery being used at the 20 hour rate (the amount of current required to discharge the battery 100%) will have "more power available" (higher AMP*HOUR rating) than if the same battery is used at (for example) the 1 hour rate.

Generally, the 20 Hour Rate (Amp*Hour rating * 1/20) is a good compromise for battery efficiency, life, wear & tear and cost. Closer to 1/10--and the battery delivers less power. More than 1/30+ rate--and the batteries are just oversized for the requirements and need larger solar panels to properly charge.

Read the Battery FAQ here, and if you want a more technical explanation, read about the Peukert Effect.

http://www.windsun.com/Batteries/Battery_FAQ.htm

There is nothing hard and fast here--but following some useful rules of thumb to initially size the system can save lots of head scratching (like sizing a battery bank for 6x the expected daily load--without sun--basically gives you 3 days of no-sun and discharges the battery to a maximum of 50% level--for longer life).

-Bill

I was wondering why all residential off grid systems are kind of lower voltage. I know gridtie run high volt systems. But kind of curious by batteries do not. I mean connecting batteries together to make a 120volt bank would be easy,

Obviously you would need an inverter handling this charge.
Maybe Solar Guppy can chime in as he has a good knowledge of the practical design of the internals.

I would imagine that it is not a problem with inverters as gridtie have high dc voltage values.

That leads me to believe it is a battery problem??? Just the amount of batteries need to tie them together or an actual problem with battery banks as voltage increase.

I have had a small off grid system with 2 100AH Deka AGM batteries about 2 years. I started with 12V (batteries in parallel) system, but now when I have added more panels the currents got so big that I changed it to 24V system (batteries in series). My array is now abotu 1600W nominal, consisting of few different vendor panels. It is wired as 24V nominal array. As a charge controller I'm using Outback MX60.

After the first year I noticed that batteries performance degraded quite significantly, but the second year I have not noticed that much degradation. When you are considering battery performance remember that the batteries will lose their performance from the start, some faster, some slower.

I had the same questions as you that should I invest in more batteries, or just keep it as minimum. I came into conclusion that the batteries are so expensive that I do not want to spend a fortune on them... since they will age anyway, no matter how well they are taken care.

At this time I do not need long backup time since my primary purpose is to use the solar power to run couple window AC units. (I live in Texas... ) I purchased a small PIC (Microchip) based microcontroller board that I programmed to control my inverter and loads based on battery voltage. When battery voltage increase above certain limit the controller turns on my inverter and turns on window AC units one at a time. (In the morning it turns on one only, and when the sun gets higher the battery charge level increases again, and controller turns on the second AC unit.) When sun starts to set, or clouds come by, this sequence is reversed. So far I have had this working only a week... but so far so good. The central AC does not need to work that hard that it used to couple years back.

Basically unless you do grid tie, or use big battery bank, you need to be pretty creative how to utilize all the solar power...