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When we got our Hawk shell, we were pretty clueless about camper batteries and how to manage them. Only through research, experience, and web sites such as an extremely helpful Four Wheel Camper Forum on a web site called Wander the West, were we able to learn the basics. At least well enough to not destroy our battery too soon! A book called "Mangaging 12 Volts" by Harold Barre, while a little dated, was also very helpful in understanding the basics.
This page covers the basics we learned about our battery and 12 volt system, including voltage changes that we monitored directly. Your system may be quite different, (for example, with heavier wiring) so you may get different results. Caution: If you are battery-experienced, this will be old stuff and probably not worth your time.
5 things this newbie learned about our 12v system after delivery of our FWC Hawk Shell
5 things this newbie learned about our 12v system after delivery of our FWC Hawk Shell
- Your AGM battery is only useable from about 12.75v to 12.8v down to 12.25v
You should not use your AGM/lead acid battery below about 12.25v as you will shorten it's life. This refers to the "resting voltage". You must let it sit without charging or using a load device to let it settle down to get an accurate resting voltage. I made a chart to elucidate the voltage vs state of charge (to the left), based on a full charge of 12.75v on my AGM battery and many sources and forums that say you should not go below 12.25v as it will damage your battery. I try to not let ours go much below 12.4v. Note, the equation on the graph, basically just says your battery state of charge changes by 20% of its capacity for each voltage change of 0.1volt.
2. Only 50% of the Amp-Hour rating for your battery is useable if you have an AGM type battery.
2. Only 50% of the Amp-Hour rating for your battery is useable if you have an AGM type battery.
Throughout the useable range identified in #1 above, i.e. from 12.25v to 12.75v, only half the total amp-hrs that your battery is rated for are used. For our factory 79AH battery, only 39.5 AH are useable, or are used in this 0.5 volt range from 12.25 to 12.75v. The rest of the energy is still there, inside your battery, but it is in the usage region below 12.25v where your battery life may be shortened. Below that the voltage begins drops faster and irreversibly until it dies, somewhere below 11.9v. I have not experienced this but it is my understanding. Note, some say you can go lower than 12.25v and the slightly shorter battery life is worth the extra capacity (>50% of rated capacity). I like to stay on the conservative side, but I know I can go to perhaps 12.0v or so in a rare emergency.
For lithium batteries (e.g. LiFePO4), you can use much more of the total charge, I believe up to 80 or 90% of it. So a 100 amp-hour lithium battery is good for 80 - 90 amp-hours of useable energy. They cost more, but are much lighter so you should consider them if you are big energy user, or camp or live where the sun is scarce, like the Pacific NW or areas with deep forest canopies. Lithium batteries can't be charged when it is below 32F, so for a cold/winter climate they have battery heaters and charge controllers to handle that. For now, for us and our climate, We think that is too much to go wrong, but we are open to reconsider, especially if we need more power or need to lighten up in the future.
3. Don't rely on battery voltage readings while it is under load or being charged.
3. Don't rely on battery voltage readings while it is under load or being charged.
After charging it takes hours to settle down.
After charging it takes hours to settle down.
This chart shows how I started with a voltage of 12.59v then charged it for 2 hours with my 100w panel (yellow shaded area). The voltage rose to 13.35v but after disconnecting the solar charger it dropped rapidly. This is a "surface charge" and doesn't really reflect the true charge on the battery. You might get a good idea after about 7 or 8 hours, but it really takes closer to 24 hours to settle down to a good resting voltage reading of 12.74v. Thus my 100w panel added 0.15 v or about 30% of a full charge in two hours. This is not a huge deal because I know that a sunny day and driving time with the alternator charging will easily give me an 80 - 90% charge at sunset, enough to last until sunrise.
After turning off a load, wait 20-40 minutes
After turning off a load, wait 20-40 minutes
This chart shows that while a device is running, the battery voltage reading is suppressed and it takes about 20 to 40 minutes after the load is turned off to return to a stable reading. The amount of suppression and time to return to a stable reading is greater for a larger load. This chart is based on a 1 amp load. You can certainly get a rough idea after about 10 - 15 minutes.
This "low" reading is not as severe as the "high" reading after charging. As you can see by the scale above, the voltage only changes 0.02v in the first 10 minutes so you can still get a good approximation pretty quick.
4. Because of #3 above, a battery monitor that tracks amps flowing in and out is a more reliable way to get an accurate picture of your remaining battery capacity.
4. Because of #3 above, a battery monitor that tracks amps flowing in and out is a more reliable way to get an accurate picture of your remaining battery capacity.
For our purposes, a simple inexpensive battery voltage indicator gives us an adequate indication of charge because I recognize the issues discussed above and I account for them when managing my battery. However, if you can afford it, you might want to consider a more capable battery monitor that tracks amps flowing into and out of the battery and doesn't rely only on voltage. Victron and Renogy and others make such monitors. If you can't afford it, you can do just fine with voltage readings, in my opinion. I'm a nerd and like accurate readings, but I'm also budget conscious.
5. Your truck alternator can supplement your battery charging when the sun doesn't shine.
5. Your truck alternator can supplement your battery charging when the sun doesn't shine.
As you surely know by now, your truck alternator is wired to charge your battery as you drive. For a true minimalist with no refrigerator who drives nearly every day, this may be enough. It is wired through a relay that assures your camper system will not drain your truck battery leaving you stranded. We recently camped for 2 days in a dense forest and our battery got down to our preferred lower limit of 12.25v, so I idled the truck for 15 minutes and gained about 0.05v or about 10% of additional capacity-just enough to last until we left. However, if we had a heavier wire ( Say 2 to 6 gauge) running from the alternator to the camper battery, we should get even more amp-hours into our battery with the 15 minutes of idling.
There are many blogs and info about upgrading the gauge of the wire running from the alternator to your camper (see e.g., Wander the West Four Wheel Camper Forum) and I think you can probably request this upgrade on your new camper install from the factory. It is an added expense as it involves additional electronics, but I recommend it if you are at that that stage in your camper acquisition. This is probably the next upgrade we are seriously considering on our rig. We'll post our experience if we do that.
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