Coach Batteries - How to get the most out of them

DC Voltmeter and Ammeter to monitor the coach batteries.
I added this to improve upon the Roadtrek LED battery indicator.
If left on, it also provides a power reading and energy consumed.

Roadtrek L-F-G-C battery display -
4 LEDs lit indicates charging or fully charged.

The post points out what I've learned  about my AGM batteries including limitations of the L-F-G-C  battery display which is simple to read but can be misleading, as I discovered.

How do we use the Roadtrek if we're not in a campground? This is important because it influenced our battery decision. We do overnight in the Roadtrek off of the grid, but we don't do a lot of boondocking. If the temperatures are above freezing but cold we may run the generator and the heat pump. However, G and I have also slept in the Roadtrek in sub-freezing temperatures while off the grid and below the limit for the heat pump. We will then run the propane furnace if necessary and that requires 12VDC.  If temperatures dip into mild sub-freezing overnight and we haven't winterized we will run the hot water heater on propane and the furnace. If we are on propane I don't think it is practical to run the generator continuously overnight and we usually don't. We rely on battery power.

What I discovered. I did not understand the limitations of that L-F-G-C Battery indicator and learned that "F" or "Fair" is really "Poor" because the batteries may be nearing depletion at that point.

I also learned that the Inverter Off-On switch didn't always turn off the inverter function of the Tripp Lite inverter/charger; that was a switch malfunction. It caused unexpected battery depletion.

I realized I had been mis-using my batteries. I didn't understand that the entire 220Ah or rated battery capacity is not available unless I want to seriously reduce the lifespan of the batteries. I also learned that repeatedly using the batteries below the "F" Fair indicator and until only the "L" or "Low" indicator was the only one illuminated was not good for the batteries, if I want longer battery life. My lack of understanding coupled with an inverter switch problem led to the early demise of my first set of AGM batteries.

I've learned about AGM batteries and when I replaced them I also investigated Lithium-Ion batteries (LiFePO4). I'll point out what I discovered about the AGM batteries in this post, but first, I decided that I would continue with AGMs because:

• 220Ah AGMs are sufficient for my use (approximately 110Ah to 170Ah useable - explained in this post).
• I have an Onan generator which I can use to provide power for coach appliances and charging.
• I was concerned about the low temperature charging restrictions of LiFePo4 batteries because my AGMs are mounted outside in the rear of the coach. That is the location I would use for LiFePO4 replacements.
• I did not want to open my wallet for the more costly LiFePO4 batteries.
• I decided to add a small 50W portable solar panel with de-sulfating 180W controller to help charging of the batteries. I concluded this was a better use of my money as compared to the LiFePO4 batteries. I may eventually put a 100W panel on the roof.
• I decided that adding a good DC voltmeter/ammeter to monitor the batteries would assist me in using them and avoid the earlier problems.
• There have been improvements in technology, and who knows, I may change my mind in 2-5 years.

This post is about a few things that may help you improve the life of your AGM batteries. 
I include a typical 12VDC energy audit at the end of this post.

My 2013 210P Roadtrek has two 6-Volt AGM lead-acid sealed batteries rated 220Ah. I use these on our treks in both warm and cold weather. We don't boondock a lot, but we do spend overnights off the grid. The numbers in this post are with those batteries in mind.

Some things I have learned
These lead-acid batteries can provide 4-6 years of good life, and sometimes more. But how they are used is a significant factor and determines how long they provide good service. Here are a few things to be aware of, and more on what the indicator in the Roadtrek is attempting to display and how it relates to what we observe with our batteries.

I don't think it is possible to reverse the aging of these batteries. It is possible to damage them.

How the batteries are used and discharged/charged is probably the single greatest determinant of how long these lead-acid batteries can provide good service. Battery life is determined by age and these cycles. We sometimes don’t get the life out of the batteries we may expect. I have two 6-volt AGM batteries. They are wired in series to provide 12V DC and can provide a maximum of about 220Ah (Ampere-hours).

What are DC Watts and Volts and Amperes?
These are important because they relate directly to our batteries. If we know the amperes our appliances and lighting are using we can estimate how long on a charge we can run them.

You can skip this and move on to the next section, but I think you will want to read this before you do an audit of your DC appliances and get a better idea of how much battery power you are using.

1. Watts are Volts multiplied by Amperes. In our Roadtreks with 12V AGM batteries:

• 12V x Amperes = Watts; for example 12V x 5A = 60 Watts.

The battery voltage does stray but for doing calculations using 12V is handy.

1. Because we know watts and the voltage, we can calculate Amperes. For example:

• Watts divided by volts = amperes. A 100 watt DC appliance: 100W/12V = 8.33A

An Ampere-hour (Ah) is a measure of amperes used per hour. Batteries are rated in Ampere-hours.

• An Ampere-hour is using one ampere continuously for an hour: 1A x 1 hour = 1 Ah

Ampere-hours let us determine how much battery capacity is being used. For example, let's assume we three things that are using DC electricity:

• 1.0A lights
• 0.8A charging phone
• 2.6A furnace and fan.

If we add these up, we can see how many Ampere-hours they would use in one hour:

• 1.0 + 0.8 + 2.6 = 4.4A
• 4.4A x 1 hour = 4.4Ah

If we multiply by the number of hours these appliances and lights are "on" we can determine how many Ah will be used in total, and as you will see this is useful for estimating how long our batteries will provide us with electricity between charges.

For example, let's say we left those appliances and lights on overnight, for 8 hours. Here is how much battery capacity they would use:

• 4.4A x 8 hours = 35.2 Ah

Later in this post I provide some estimates which can be used to determine how long your batteries could last when providing DC power.

What use is knowing Ampere-hours? Ampere-hour (Ah) rating of a battery is a measure of how many amperes a battery can provide when discharging. One Ampere-hour is one ampere for one hour.

Unfortunately, a battery rated 220 Ah cannot provide 220 Amperes continuously for one hour.  In watts that is 220 A x 12V = 2640 Watts total.

A 220 Ah AGM battery cannot be used that way.  The more we attempt to pull from our batteries, the less Ah they can provide in a short period of time. I'm using the data provided by the manufacturer of my batteries, which is relative capacity. Relative capacity takes into account "battery fade" which normally occur to batteries as they age and are used:

1. To get good battery life, these batteries should not be repeatedly discharged more than about 50%, and when discharged should be immediately recharged. In other words, one cannot get 220Ah out of these batteries if we want good life from them.
2. A 50% discharge of 220Ah is 110Ah that we can use. The batteries must then be recharged. AGM batteries discharged repeatedly 50% and then recharged can provide about 1200 charge-discharge cycles. That’s the service life. Keeping batteries fully charged and not storing them partially discharged also is an aid to improved service life. Temperature also impacts service life, and temperatures above 77F reduce battery life.
3. These batteries can be discharged repeatedly by 80% but that will reduce the life. A 80% discharge of 220Ah is 176Ah that we can use. AGM batteries repeatedly discharged 80% can provide 700 charge-discharge cycles. That is much lower than the number of cycles if we only discharge to 50%.
4. Many battery manufacturers use a 20 hour rating for batteries. That is a more realistic and useful measure of battery capacity. For example, my AGM batteries are rated 220Ah, which implies continuous 220A for one hour. The actual 1-hour rating of the batteries is only 130A.
5. The battery can, however provide a total of 220Ah over 20 hours of discharge.  That is a continuous 11 amperes for 20 hours.  But we should only go for 50% which is 5.5 A for 20 hours. This is to get better battery life. 
6. Over 10 hours, a typical night, the battery can provide a total of 210Ah according to my battery manufacturer. In other words it can provide 210Ah/ 10 hours = 21 amperes continuously each hour for that 10 hours. But, if we only discharge the battery 50%, of the 220Ah full rating, that implies about 110Ah/ 10 hours or only 11.0 amperes continuous discharge for 10 hours.
   
   Conclusion: Over a typical night, my batteries could provide 11.0 Amperes each hour and provide good service life.
7. These are the characteristics of the batteries in my Roadtrek when they were new. This capacity diminishes with battery age and use. 
8. There are differences between manufacturers, so it is best to check the specifications for your batteries.

What are cycles?

1. What does 1200 charge-discharge cycles mean? That's the number of times we can discharge and immediately recharge the batteries. For example, if you discharged the batteries to 50% and then fully recharged them once a day and every day, they could have a useable lifespan of 1200 days. 1200 days/365 days per year= 3.2 years battery life.
2. If you did this every three days, then we would complete a cycle every three days, or 365/3 days = 122 cycles in a year.  1200 cycles/122 cycles per year = 9.8 years life.
3. However, because of other factors including temperature, length of time the batteries sit in a partially discharged state, battery age, etc. it is unlikely we will ever achieve this in our Roadtreks.  The manufacturer’s battery data is based upon ideal situations, including a temperature of 77F

What occurs as batteries age? As batteries age there are internal changes and that reduces the capacity. For example, after 600 cycles of charging and discharging the battery to 50% what occurs?

1. As the batteries age, even under ideal conditions, we may not get that 110Ah because the capacity of the battery diminishes as it ages. This is called battery fade. In other words the battery initially can provide 220Ah, but the actual capacity decreases over time.  We notice this as a more rapidly falling terminal voltage, which we can see on the Roadtrek L-F-G-C display, which spends less time in the “Good" area. In other words, the display falls from C to G to F more rapidly than one would expect. That is an indicator of aging batteries with diminished capacity. Eventually we decide that the batteries don’t provide us with enough power to get through the night or whatever while powering our devices. We then get new batteries.

What happens if we repeatedly discharge the batteries below 50%?

1. Frequently discharging the batteries below 50% will further reduce the service life. For example, we can repeatedly discharge them to 80% (20% remaining). If we do this,  the service life will decrease. My battery manufacturer states that discharging repeatedly 80% will reduce battery life to about 700 cycles. That is considered to be the lowest acceptable service life.
2. At 700 cycles if we discharge and charge every 3 days (122 times a year), the batteries will have a useful life of about 700 cycles/122 cycles per year = 5.7 years. But we must also consider the aging of the battery, temperature and so on. These also reduce battery life.

There is a trade-off.

1. We must decide between longer battery life, or more power from each charge, or a compromise. In practical terms we must choose between how long we want to power our DC appliances each time we discharge the batteries and how long a battery service life we want. We can’t get both maximum power for maximum time because the deeper we repeatedly discharge the batteries the shorter the service life. We notice this as how quickly the available power diminishes. Of course, we can replace the batteries every three years or so. That is a financial decision.

Does the Roadtrek “L-F-G-C” tell me when the battery is at 50%? The information I have about this is the “G” indicator is “ON” if the battery in my coach is above 11.9 volts, which is about 29% state of charge for my batteries.

1. If I use the "G" indicator and recharge the batteries when it goes out, then my battery manufacturer indicates that I can get about 800 cycles from my batteries if I discharge them to about 20-30% repeatedly.
2. Important Note: I have not verified the Roadtrek indicator with actual measurements comparing the battery voltage to the indicator LED thresholds (I use a voltmeter and no longer pay much attention to the indicator). I'm using published information and it is my understanding that the “F” indicator is “ON” if the battery is above 11.2V,  but the table for my batteries indicates a 0% Relative state of charge when the voltage decreases to 11.6V and completely depleted at 10.5 volts. The table provided by the manufacturer of the AGMs in my Roadtrek indicates 50% relative State of Charge = 12.35V and 100% State of Charge = 12.9 volts.  I’ve seen other AGM charts and those batteries were in the range of 11.66V (20%) to 12.05V (50%) to 13.0V (100%).

What else should I know about the Roadtrek “L-F-G-C” indicator?

1. It measures battery voltage. This is an approximate indicator. A battery will give two different voltage readings. One reading if it is being discharged; the another is if it has rested (no discharge) for about 6 hours.  The battery specification for “State of Charge” are usually for a resting battery. If a battery is discharged and then allowed to rest the voltage will usually increase. In other words, the battery may have more capacity remaining than the “L-F-G-C” indicator represents when we are in our Roadtrek and discharging the coach batteries.

What is the best approach for the batteries?

1. Use a voltmeter if we want a better idea of the condition of the batteries, so we avoid excessively discharging them. Avoid high temperatures because battery life decreases at higher temperatures.  Measured  life is usually at 77F and many batteries will lose half of their life if the temperature is 95F. Don’t charge if above 120F. Charge the batteries after every period of use. Don’t discharge more than 50%. All of these things improve the life of the batteries.
2. We can add a voltmeter in the rear cabinet of a 210P to monitor the voltage. 

3. 

   12VDC socket in the rear of my 210P

   
   

   

   Inexpensive 12VDC digital meter
   plugs into socket in photo above

   

   Inexpensive 10 inch Splitter Cable for 12VDC
   Use if we need two devices from a single 12VDC source,
   such as the photo of the cabinet, above. 

   
   

   

   Dual, fused splitter with digital voltmeter
   and USB sockets

   
   
   As battery capacity diminishes what does that mean in practical terms if I want to boondock? 
   
   1. There is a trade-off. Longer battery life, or obtaining more power from each charge. This is further complicated because as the batteries age, their capacity is diminished. The actual capacity of a 200Ah battery will gradually decrease to 190 Ah, 180 Ah, 170 Ah and so on. This is because the internals of the battery change as it ages. We usually notice this change because the voltage decreases more rapidly as the battery discharges. 
   2. The beginning voltage of a fully charged battery will be that of the charger which is about 14.7 volts, this is called “surface charge” and this charge dissipates quickly as we discharge the battery. Both good and faded batteries will usually show a "C" when on the charger, and immediately after they are disconnected.  That is deceiving and is not indicative of the actual state of the batteries.
   3. If an older 220Ah battery has a capacity that begins at 160Ah capacity then it won’t take very long to get to that 110Ah level which is the 50% capacity level of the new battery. We see this as a more rapid movement of the indicator as the indicator moves more rapidly from “C” to “G” to “F”.

Are there other things I should be aware of?

1.  Here is the short list. Batteries must be properly charged in accordance with the manufacturer’s instructions, automatic chargers are best. For golf cart batteries at 12V that usually means an Absorptive/bulk charge at 14.7 volts and a Float charge at 13.5 Volts. AGM batteries may be slightly different with an Absorptive/bulk charge at 14.4 volts and a Float charge at 13.5 Volts. My AGMs recommend an Absorptive/bulk charge range of 14.4-14.7V and a Float charge of 13.2-13.8V. Because the lead-acid batteries are chemical devices, there are other noticeable issues. 
2. Battery capacity is reduced at lower temperatures. A 200Ah AGM battery may have a capacity of only about 80% at 32F or 160Ah.  However, that is at a discharge of about 20Ah (about 4A per hour for 5 hours).If we increase the to 40Ah, the capacity reduces to about 70% or 140Ah. (That provides a rate of about 8A per hour for 5 hours). However, we still have to watch the battery voltage to determine the actual condition, or state of charge. We can’t simply watch a clock.
3. A 200Ah AGM battery is rated to provide that amount, 200Ah over 20 hours. It can provide 200Ah/20 hours = 10 amperes per hour continuously for 20 hours. But we would not want to exceed that 50% discharge limit if we want acceptable battery life.  In practical terms, a 200Ah battery should be considered 100Ah. if that is so, it can provide 100Ah/20 hours = 5 amperes per hour continuously for 20 hours.
4. If we increase the amperes used, then the time the battery can produce it will decrease. The same batteries will provide only about 170Ah over 5 hours, not the 200Ah rated over 20 hours. In other words, the more power we demand, the total amount decreases. That’s because these are chemical devices. If the batteries could provide 170Ah over 5 hours, that’s 170/5 or 34 amperes each hour at 100%. But if we allow only 50% discharge, we then can use only 34A x0.5 = 17 amperes per hour over the 5 hours. 17 amperes x 12 volts = 204 watts connected to the battery. 
5.  If we use an inverter to get 120VAC from our batteries, we need to take into account the inefficiency of the inverter. In other words, if we run a 200W appliance at 120VAC on the inverter, the amount of DC power going into the inverter is greater than the AC power coming out. Simply turning on the inverter uses power, perhaps as much as 200W. To maximize battery power, leave the inverter Off if we don't need AC from the batteries.  Tripp Lite suggests using 1.2 as an inefficiency multiplier:

• Begin with the AC amperes or watts of the 120V appliance. If 2.5A, then the watts are 120V x 2.5A = 300W.
• Then to determine DC amperes divide by 12V.  300W/12V = 25 DC Amperes.
• To estimate the battery Ampere-hours (Ah) required, multiply the DC Amperes x time x inefficiency.  25A x 4 hours x 1.2 =  120Ah. That’s a rough estimate and it exceeds the 50% capacity of a 200Ah battery.
• To determine amperes from an appliance using DC watts (such as a TV), simply divide the watts by 12 volts.  For example for 100 watts: 100W/12V = 8.33 A

How can I recharge my batteries? The sources of electrical energy for recharging are determined by what’s installed in your Roadtrek. These may include the following:

• Vehicle engine.
• Shore power.
• Onan generator.
• Underhood generator (GRU) - I don't have this.
• Solar panel.

How long does it take to re-charge batteries?
The more depleted the batteries, the longer the charging time required. 

• Using 120VAC and the Tripp Lite 750 Watt Power-Verter DC-to-AC Inverter/Charger, it can take 12 hours to completely recharge discharged coach batteries. This was confirmed by a factory technician. Furthermore, the charger has two settings: 10A and 45A.
• Using the Onan generator is the same as shore power because the Tripp Lite is used.
• Solar may take longer. This is determined by the amperes available from the solar panels. If a 45A Tripp Lite requires up to 12 hours, it is reasonable to assume solar may require more time because a 200W solar panel is providing about 17A. However, most RVs on solar don't draw down the batteries because during daylight hours the solar provides a part or all of the DC power requirements of the RV.

I need a 12V DC power budget.  To do this we need to do an energy audit. That’s a way to determine how far my batteries can go before I run out of DC power, and then have to recharge

1. To get maximum power available and maximize your batteries, it is better to use 12VDC appliances than 120VAC connected to the inverter because of the inverter losses. 
2. Add up the DC amperes to determine how much you use. Then calculate the Ampere hours to see how long your batteries will last before requiring a recharge.
3. Add up the ampere requirements of all of the DC appliances and things in your RV. Product labels are reliable sources. Because there are differing options, I can't provide the Amperes required for each and every device. But I do provide a list with some approximations if I know what they are:

• Lighting 12VDC (LED uses less DC energy than bulbs and fluorescent) - Varies.
• Each LED light may require 0.25A.
• Lighting 12VDC fluorescent, single 20W bulb - 1.3 to 1.8A. 
• Lighting 12VDC fluorescent, dual 20W bulbs - 1.8 to 2.2A. 
• Propane detector 12VDC about 0.1A.
• 12VDC TV (about 32W or 2.7A).
• Amplified TV antenna - varies.Macerator 17A 12VDC when running.
• Water pump (varies with pressure and flow, possibly 5A 12VDC when running).
• Inverter 2A losses or more if ON.
• Roof fan - Fantastic Fan 3350 - 2.5A when running (varies with speed).
• Bathroom fan.
• 3-way Refrigerator 12VDC for controls - approximately 1 to 2A.
• 3-way Refrigerator on 12VDC cooling - approximately 20A.
• Dometic LCD single zone thermostat.
• 16,000 BTU Suburban Propane Furnace when running - 2.8A.
• 12,000 BTU Suburban Propane water heater - 12VDC Module board - 1.0A.
• 15 inch Laptop - estimated 90 watts 120 VAC.
• Smartphone charging - estimated 5 Watts @120V,  0.5A @ 12VDC.
• CPAP machine - varies.

(c) Copyright 2019 Norman Retzke "All Rights Reserved".  See disclaimer notices.