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Battery Management Overview

Battery solar

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#11 PaulT

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Posted 16 February 2016 - 07:58 AM

Everybody, thanks for all the great info. I’m currently reading through the links you sent, which will probably result in more questions. Until I get through them all, I've responded to everyone below.
 
@PaulT
All this is great. I’m pretty sure I get what you’re saying. Bear with me, and let me summarize in my own words to see if I’ve got it right:

  • Besides the Zamp’s main functionality as a charge controller, I can roughly use it to gauge the current state of the battery but without the exact specs a true Trimetric would provide.
  • Got it. Basically, Amp Hours is how much energy I have to use in my batteries. From what I’ve read as well, you never want to drain more than 50% of the batteries. Correct?
  • Instantaneous Voltage is the reading of the batteries is at the current state. Instantaneous Amps is how much current is being generated by the solar panels at that moment. I understand that Amp/hrs is how much energy is stored in my batteries, but does the Amp/hrs reading on the Zamp controller indicate how much energy is being put back into the batteries?
  • If Voltage is a poor approximation of the SOC, should I pay much attention to the Voltage reading on the controller?
  • Is there a do not dip below this number? I think I read if you dip below 12.2 Volts, that’s no bueno.
 

1. You can get an approximation of SOC from the Zamp
2. Essentially correct but that is 50% of amp hrs, not 50% of voltage. Here is where the Trimetric has an advantage.
3. As near as I can tell, the Zamp amp hrs indicates how much solar generated energy has been sent to batteries but does not include energy from truck or shore power nor does it track power consumed from battery.
4. Voltage does give some indication of SOC but be aware that indicated voltage will drop when a power draw like fridge is happening & rise after the power draw stops. This makes judging SOC from voltage iffy.
5. If the charge gets too low, you may have to turn off the loads to allow the solar to bring the voltage up enough to allow the battery separator to connect to and charge from the truck. And, yes, you can shorten battery life by too deep discharge.

Paul
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#12 MarkBC

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Posted 16 February 2016 - 05:39 PM

@MarkBC

I speak fluent analogies. Thanks Mark. I understand the basic difference between Amps and Amp/hours, but am still unsure of when to pay attention to one versus the other. You got another analogy up your sleeve?

 

 

When to pay attention to amps vs amp-hours?  Well...it depends on what you want to know.

Another analogy?  OK, how about this one, based on motion:  Amps are like speed -- how fast you're moving.  Amp-hours are like distance -- how far you've gone or how far you have yet to go.  You divide distance by speed to get the time it'll take to travel the distance.  And you divide amp-hours by amps to get time to empty (at that discharge rate).   And both quantities are needed to figure that out.

 

I think both quantities are important...but I guess I might say that amp-hours are what you should pay most attention to, just as (yet another analogy) "I have 1/8 tank of gasoline left in the truck's tank" is probably more useful to know than "I'm using gasoline at 1 cup per minute".

 

But, as Paul points out, that meter doesn't directly measure how much energy (amp-hours) are in the battery.  So, you may not really know how many amp-hours of energy you have in the battery.

 

So, what do I pay attention to with my system (240 watts of solar feeding into two 130-amp-hour batteries)?  Well, I don't have a meter that monitors energy in and energy out, my simple meter only measures what the solar panels are feeding and have fed the batteries, so I don't have any kind of measure of amp-hours in the battery.

I pay attention to battery voltage.  Yep, the voltage can be misleading -- even completely wrong -- when my solar panels are charging the battery and similarly misleading when my refrigerator is pulling several amps from the battery.  But I'm aware of that and can still get useful, if approximate, information about "how full" my battery is, based on voltage.

  • If it's late at night or pre-dawn in the morning, it's been hours since the batteries were receiving solar charge, and if it's been a while since my fridge has been running, then the battery voltage on the meter is a good approximate indication of the battery charge.
  • Even when my solar panels are charging the battery I can still get useful information:  If the charge controller light is steady on then it's in bulk charge stage --  a ways to go before full.  If the light is blinking slowly then it's in absorption stage -- getting up there.  If it's blinking fast then it's in float stage, battery full or close to it.

Of course, this is only approximate...but I don't need high accuracy.  In that sense it's not like the gas tank analogy -- the consequences of "running out" aren't nearly as bad for my camper electricity as it is for truck fuel when driving in the middle of nowhere.

And I can always run the engine and drive if I need to charge the camper batteries.

 

(By the way, that "amp-hr ÷ amps = time" calculation is only theoretical. And rate of discharge affects the amount of electricity that's available.  That is, the higher amps you're pulling from a battery the less total amp-hours you'll be able to pull.  It's kinda like driving faster reduces gas mileage, so you can't go as far on a tank of gas when you're driving fast.)


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#13 PaulT

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Posted 16 February 2016 - 06:45 PM

+1 on the analogy to the fuel tank with the caveat that you don't reduce the fuel tank capacity by running it too low.multiple times.

The liquid analogy simulates the electrical system only so far. :P I started to use it in my reply to OP but have been pounded around the ears too many times by more pedantic electrical engineer co-workers after using it.  :wacko:

 

Paul 


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#14 ntsqd

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Posted 17 February 2016 - 02:02 AM

I too have been subjected to that by my EE co-workers. So now I use it exclusively in their presence. :)


Edited by ntsqd, 17 February 2016 - 02:03 AM.

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#15 bignerdski

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Posted 17 February 2016 - 06:12 AM

Alright, I think I have the basics understood well enough to put theory into practice this weekend.

 

Thanks for the further clarification @PaulT and @MarkBC.


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#16 northshorehenry

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Posted 17 February 2016 - 06:43 AM

And you know what? It's ok to make a mistake. We have all left a porch light on now and then, ran the battery low, and oops. No different then if you left your head lights on. Fortunately, if you ran your house battery low, you can still get in your truck and drive away. Experiment a little the first few nights. Perhaps go to a place where shore power is available. Run your camper for an entire night with the furnace on and see where the battery ends up, the shore power in the morning will make your life easier. When you have a good idea of what your system is capable of, then start the boondocking experience.


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#17 Vic

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Posted 17 February 2016 - 01:28 PM

5. If the charge gets too low, you may have to turn off the loads to allow the solar to bring the voltage up enough to allow the battery separator to connect to and charge from the truck. 

Paul

 

This is a great thread - I have almost the exact same set-up (but w/ 2 6 volts) and am trying to understand it better.  

 

This one really caught my attention as I had never considered this and need to research this further.  Now that i look back I may have had this exact problem on a prior trip.  Now I need to find the separator and see if it has any specs on it for connecting.

 

-Vic 


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