Author: The Buchanans Rolling Down The Road

What can be said, about a crusty old curmudgeon, with no fixed address … not much I guess.

Battery Basics

Solar is a mystery to so many people, what, when, why, where, and how – there seem to be more questions than answers, and every dam fool has an opinion on everything – so this is just another opinion and it may or may not apply to you but take it for what it’s worth.

I think every one needs to understand solar! And to do so you need to have a basic understanding of batteries as that is an essential part of any solar system, even your solar operated walkway lights. I know I’m an opinionated old fart so here it goes. I have spent over 40 years in the automotive world, a licensed automotive technician, and really all I knew about batteries was the more cold cranking amps the better chance you had of starting your car on a frosty morning. We tested the specific gravity of batteries with a hydrometer to see if they were charged, we tested the battery strength by setting a specific load with a carbon pile, to check the voltage under load, rough voltage reading 12 volts it should start, 8 volts won’t start and 10 volts is a crap shoot. We recharge our car batteries with an alternator and if it provides between 13 and 14.5 volts it should charge the 12 volt battery more than 14.5 would boil the acid, and for most of my life that’s what I knew. The automotive manufacture would send out battery testing equipment that could check a batteries condition electronically, but we always felt it was more of warranty cost saving venture, as there was no real loading of the battery.

Typical AGM lead acid battery

Now I know a little more about batteries (notice I said a little), I now know my coach world my batteries are based on amp/hrs and the easiest way to get my head around that was if I had a light bulb that drew one amp and I had a battery with 100 amp/hr rating it should keep the light on for 100 hours, and I would be very wrong….at best it would keep the light on for 50 hours because if we draw the battery below 50% we are damaging it. I have hurt a lot of batteries in my day…just saying. The second thing I thought was if a 12 volt battery was at 50% it would be at 6 volts….wrong again….a 12 volt battery that is at 50% still has a voltage reading near 12 volts and a battery at 100% should read something over 12 volts like maybe 12.8 volts….shut the front door. This old dog needs help, so my 100 amp/hr battery only has 50 usable amp/hrs and if I use all 50 I am damaging the battery! I’m back to I have hurt a lot of batteries….wow. So in a perfect battery world I would only use 20 amp/hrs of the declared 100….WOW. And just to really confuse this old fart every battery manufacturer is different, yeh yeh I know they are different companies but what I did not know is that each battery manufacturer had specific charging rates…like they tell you what voltage to charge each stage at, and what battery voltage to move to the next stage…well in the automotive world it either charged or it didn’t. We all have battery chargers, every electronic item has it’s chargers who knew you should not mix them up because every battery needs different charging.

There are four stages of charging batteries:

  1. Bulk
  2. Acceptance
  3. Float
  4. Equalization

And every battery manufacture tells you how to do each stage and for how long. We are going to discuss the first three as the fourth is a whole new kettle of fish….so to speak.

BULK – high amperage with low voltage – this one is fun because the numbers are bigger, our battery bank is made up of 4 AGM 224 amp/hr 6 volt batteries so a total of 448 amp/hrs at 12 volts, of which 224 amp/hrs are usable, but in a perfect world I should only use 100 amp hours. See I finally got my one amp light to be lit for 100 hours and it only took four batteries…lol. So how fast can we charge our batteries, it depends on a lot of things but the quick answer is pretty darn fast and at big numbers, our batteries will charge at over 100 amps but only for a few minutes, why you might ask if it took 100 hrs of my one amp light to discharge my battery and I can charge them at 100 amps it should only take one hour right? Well No that’s not quite right if we took out one hundred amps it’s not that quick and simple, every battery has a resistance to giving and taking amperage, remember it is a chemical reaction that creates the amperage that has to be reversed to put it back, when you start your car you pull large amperage for a very short period of time, that’s why starting batteries are rated in cranking amps, our chassis batteries are 800 CCA (cold cranking amperage) each so we have up to 1600 amps to start the coach and the starter probably draws 600 amps for a few seconds so the battery is designed to release amperage quickly while the deep cycle type used for our battery bank releases amperage slowly which means it accepts amperage from the charge source slowly as well.

So we need to be very careful with our charger, ever wonder why there are expensive and cheap battery chargers that both say they are 50 amp chargers, well I didn’t until very recently. Then I find out that most of us are not charging our batteries fully or properly. Any coach that has a battery must have a way to charge it, so how are we doing that, a separate charger / converter hidden under some cabinet that charges the battery when hooked to shore power, or it could be part of the inverter / charger that we use when not connected to shore power, so when we plug into shore power it charges the batteries, or we may charge the batteries with our alternator when driving by using a rectifier to separate coach and chassis batteries and charge both, or with a solar controller if the coach is equipped.

I know now we have even more questions than when we started, well here is what 24 months have taught me, a battery monitoring system is a necessity, if you don’t know what condition your batteries are in how can you take proper care of them? The monitor will tell you if the batteries are at 85% or 80% and how many amp/hrs from fully charged they are, it also becomes your starting point as to the draw of every item on your coach, there are draws on the system at all times, the control board on our refrigerator draws a little more than one amp and it never quits, so 100 hrs and we are 100 amp/hrs down, and I am willing to bet that most of the batteries in most of the coaches have not been fully charged since they were installed if they were fully charged then. Our coach was almost 20 years old and did not have a monitor…I know now how the batteries are doing, what my draw loads are, even how many days since the batteries where fully charged. And being fully charged is hard to obtain, real hard, why so hard you ask because to takes hours of low amperage, high voltage charging to obtain and most chargers struggle to get there.

ACCEPTANCE – this is the charging stage that we are referring to low amperage, high voltage and seem to take forever! And after we have put every amp/hr back into the batteries we can say they are fully charged. Only now we can go to the third stage of charging.

FLOAT – this is some times called maintenance low amperage, low voltage to just keep it fully charged

EQUALIZATION – this is the “Hail Mary” of the charging world, used only after you have failed to keep you batteries fully charged. If lead acid batteries are not kept fully charged the lead plates sulphate this is part of the chemical reaction that discharges and recharges our batteries between the lead plates and the sulphuric acid, this is done with very high voltage and most chargers can not achieve this level and most battery manufacturers do not recommend this process, it does work, I have done it and are still using the batteries.

I know this is getting real long so the last subject for this part is on mixing batteries, do not do it. The battery bank is only as good as its weakest link, batteries have a life expectancy, the will only survive so many cycles, putting a new battery into the bank will help the bank but only to the level of the next weakest battery, just don’t do it!

Off Grid – Part 1

OK lets get started, this shouldn’t take more than a month or so to figure out…like seriously how hard can it be? All we want to do is to live for a couple of months in the Arizona Desert boondocking. Because we are doing a major upgrade to Thunders electrical we have to determine how much power we need, how much battery we need to supply it and of course how we can recharge to do it all over again.

So we have to start by looking at our normal daily usage of power, yes I know that we have items that draw huge amounts of power but how often do you use them and for how long?

If boondocking today I would get up, and my first stop would be at the control center to check the batteries, percentage and voltage. If satisfactory I would turn on the inverter which would start all our iPads and phones to start charging and I would have power at the cooktop to ignite the propane to perk a pot of coffee. The other thing that happens is that we also start every watt sucking phantom load item in the coach…like our satellite receiver, the dish, the television, the Blu-ray player, the microwave, you get the idea. It is all those little draws that add up over a 24 hour day. Notice I said that I had to turn the invertor on, that’s because we had to shut it down the night before when we went to bed, Why? you night ask because the draw over night might be enough to damage the batteries or set of low voltage items like the smoke / propane monitor.

Every thing in our coach wants a little power, our minimum draw with everything shut off there is about a 2 amp draw, it is made up of a number of things like the DC control board on the refrigerator and all the little things that monitor our coach. But that 2 amp draw for 24 hours is 48 amp hours of battery. And with the invertor turned on the draw jumps up to 5.5 amps and if you figure that for 24 hours, now we are looking at 132 amps of battery power every day that we have to replace. So how do we go about determining what we need?

We must convert it all to one common denominator some items give us power consumption in watts some in amp some are 120 AC and some are 12 DC so we have to convert every thing to 12 volt DC because that will be our source for the power supply our battery bank. we will use ohms law to convert every thing so that we can determine the size of inverter we need which will determine how big our battery bank needs to be and last but not least how much solar we will need to recharge the batteries.

Here is how Wikipedia describes Ohms law:

Ohms

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I=V/R, where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms.

Ya ya I know its all Greek to you so I will try to make it simpler for you if we know any two numbers we can calculate the third lets use the microwave as an example we know it plugs into 120 Volts and the plate on it says it’s 1000 Watts so we need to know the amps so take the (W)atts  1000 and divide by the (V)olts 120 and that will give us (A)mps of 8.3 of AC current.

And we will run the microwave for 1 hour to cook dinner so we will have used 1000 watt/hours.

Now we must convert to DC amp/hours because we use a 12 volt battery bank so that 1000 watt/hours divided by 12 volts of the battery bank equals 83.3 amp/hours.

So in a perfect world a single 100 amp/hour lithium battery should feed a 1000 watt inverter for one hour to run the microwave to cook our dinner.

kill-0-watt

Now that is not reality but it starts the process, so what I will do is list every thing in our coach that draws power both 120 volt and 12 volt. Then we will assign an educated (from experience) estimate as to how long each is ran every day like how long does the TV run per day, how long to charge your phone per day, your lap top, your fan, your toaster, your residential fridge, just track a few days of use and see how many watt/hours. Some items are difficult like the residential fridge because the compressor only runs part time I would use a device called a Kill-O-Watt meter, it plugs into the wall socket and the you plug the device into it and leave it for a set time and it will track how many watts in total were used and I like a 24 hour time so it is much truer picture of the power usage.

So here is your project list all your electrical, remember to separate by voltage and on part two we will start adding up the watt/hours. You can sometimes find a spreadsheet that helps with the list and times and does the calculations…I will be creating one in Excel with the calculations built in and will gladly pass it along if your interested.