Charging, batteries, solar, frigs etc in one place

[booster]

There has been a lot of interest in things related to charging with solar, engine generator, Onans, etc concerning battery capacity needs and expansions. Many of the questions are from relatively new members that weren't around here when past discussions have happened, so they would have missed them. The information and discussions lately have been scattered at best, and often left with incomplete understanding or solutions for several reasons.


This forum's archives are full of lots of very detailed and documented discussions on essentially all the topics that have been coming up lately so lots of information is available for those that have an interest in the details and tech things related to them.


I figured I would just put a list here of the kind of things that have been discussed with quick and short synapses of MY opinion of it, which you can take or leave, but it would be good if you consider going back and looking at the discussions before jumping to conclusions. It is mostly related lead acid batteries, wet or AGM, and some of the things are not in line with what the "rules" we have all heard for many years about the subject.


What I would suggest is that if you see something in the list that is of interest and can't find it in the archives, just ask and there should be someone who remembers where to find the information for you. If you want something added to the list, just mention it.



I would also request that the discussion be civil as there have been some issues with that lately.


The list:


* How do you know if your batteries are full after charging? Per the manufacturers of almost all the brands, both AGM and wet, the recommended best (by a large margin) way is to charge at absorption voltage, temp corrected, to that manufacturer's spec charge amps at the end of the charge cycle. Specific gravity in wet cells isn't quite good. Voltage, even rested, is not very accurate unless you have tested actual state of charge on that battery and made a list of what the voltage was at each point.


* Do multistage smart chargers do a good job of getting batteries full, and not overcharged, when a full charge is run? In all but a very few shore chargers they do not get the batteries full without overcharging, and most can do either over or under depending on a lot of factors. In solar controllers their are a few more choices of models that will do a good job of charging correctly. There is currently, at least that I know of, no engine generator control that will not over or under charge, with the possible exception of a full auto centralized system like a Victron for lead acid batteries. I think some of the all in one lithium systems do a very good job, though.


* Why don't chargers do a good job of charging? The don't end the charging based on amps as in the first item of this list, but try to guess at the correct charge time based on timers and/or algorithms.


* How do you know if your charger is getting your battery full? You can do a capacity test by running a fixed load at a 20 hour or other rating and see if it makes spec or not, but that is very difficult and time consuming. Best is to use us a properly set, shunt based, battery monitor as it will determine if the battery got full based the charging amps as described in one and tell you. You can also manually watch the amps at charging (making sure the voltage stays at absorption) with a good ammeter to determine full, but it is a bit of PITA because the meter needs to be inline or on a shunt to be accurate. Even the best of the inductive pickups I have seen aren't really very accurate at the very low amps you might see for some battery systems.



* Will you kill a battery's life very quickly by going to lower than 50% state of charge (50% rule)? No, you won't based on the data found in past discussions. If you do it all the time to 20% SOC you would lose 15-20% of the total energy in and out the battery compared to going to 50% SOC. You will not cut the total energy in/out in half as the rule states, you just get less recharge cycles. Discharges appear to average, so one that goes to 20% gets balanced by one at 80% and is basically the same as two at 50%.


* Is the Peukert factor correct? Yes, as it is defined, it is correct, you get less amp hours out a battery at faster continuous discharge rates. Does Peukert apply to way our RVs are used? Sometimes it may, but in many (most?) cases it does not. The reason is that in an RV you almost never have one continuous discharge rate that the definition is based on. Testing done by members here indicates that the "ending" point of discharging a battery is only related to the amps being used at the point the battery hits 10.5 volts. For instance, you could discharge 1/2 the amp hours of a battery at high rate, and then cut the rate down to a lower level for the second half of the discharge and you would get the same amp hours out as if you discharged the entire amount at the lower rate.


* How does a battery monitor work and why use one? The monitors use a shunt to very accurately measure amps in and out of the battery, ignoring any van loads, and use a totalizer to add them up or reduce them to amp hours or watt hours actually in the battery. The also use the amp reading to check for the correct, set, charging amps to indicate a 100% charged battery, but most can't shut the charger off. The will give continuously visible state of charge of the battery or how many amp hours you have used. For checking how much power any device uses, you just turn the device on and see how much the amps go up on the monitor.


* Are battery monitors accurate? Yes, with a caveat. If you have a full battery based on amps, the battery monitor will calibrate itself to that full amount so the 100% is right on. If the battery still has the capacity you have set in the settings, the SOC will be correct over the entire discharge. If that capacity has changed over time, which certainly happens, your SOC % can be off by that amount. It takes a capacity test to see what the capacity is, as mentioned earlier, but that is difficult. Best if probably to check it when the batteries are new by putting in the rated capacity, run it down to 20%, let it sit a few hours with no load and look at the voltage and note it. If you run it down to to 20% in later years and that voltage has gone lower, you have lost capacity so just reset it until you again at the same as new voltage at that 20%. The 100% sets itself.


Now the inaccuracy. The monitor recalibrates to 100% on every full charge so you need to get full charges regularly to do that. On the recharge side, the charge efficiency can influence the accuracy because charge efficiency varies with depth of discharge, charge rate, and the current state of charge. Efficiency if very high when you are at 20% SOC and very low when you are at 98% SOC. The monitors can be set for charge efficiency compensation or do it themselves, but that is still a guess of what the next charge cycle will be. These errors make the recharging SOC less accurate and if you don't get full before discharging to get a 100% calibration they will accumulate and get to be significant on both discharge and recharge. This is one reason why getting a regular 100% full charge is necessary, along with increased battery life.


Can I run a compressor frig, microwave, hair dryer, cpap, whatever, on my current system? Nobody can accurately tell you that unless they know how much battery, solar, charging etc you have, and also how much power you on everything else. Put with that your camping style and needs and a decent calculation can be made. Without all that it is just a guess, which may or may not be good. If someone has a similar system and style of camping, they may get quite close, and that is where forums like this one are very useful. IMO, don't believe anything that anyone trying to sell you anything says without confirming it with non biased sources as there are very many false claims made for RV stuff every day, even in the manufacturers literature.


If I do all the good stuff will I never run out of power or kill my batteries? As long as you do the necessary watching (minimal for most) of some of the things while camping, yes, it should be safe. Of course, all of us have probably done something done like leave something running and let the van sit until it kills the battery.


HAPPY CAMPING...

[GallenH]

Thanks for the summary. Good info.

[markopolo]

Yes, great summary by Booster


He posted another great summary here re: PWM and MPPT here today: http://www.classbforum.com/forums/f2...html#post97131

[GeorgeRa]

Good post, I would think that quick explanation for shunt based SOC (State Of Charge) meter could be useful. Here it is Victron video, very good SOC meter.

https://www.youtube.com/watch?time_c...&v=MRljOaTdc3s

[Jill Ellis]

Thanks a ton for well explanation in details. it will basely help us to get information charging or batteries issue etc.

[booster]

Originally Posted by GeorgeRa

Good post, I would think that quick explanation for shunt based SOC (State Of Charge) meter could be useful. Here it is Victron video, very good SOC meter.

https://www.youtube.com/watch?time_c...&v=MRljOaTdc3s

Agree on Victron monitor, especially since it has an output that can be used to control other things. Right now it is one of the only options for shutting off engine charging without doing it manually, but has to be home built system. Only bummer is the output is very current limited so probably would need to go through low SS current relay first to control other large capacity controls.

[avanti]

Originally Posted by booster

Agree on Victron monitor, especially since it has an output that can be used to control other things. Right now it is one of the only options for shutting off engine charging without doing it manually, but has to be home built system. Only bummer is the output is very current limited so probably would need to go through low SS relay first to control other large capacity controls.

The Trimetric meter also has a serial output port that I have successfully interfaced to external logic. Definitely geek territory, though.

[GeorgeRa]

This is a new SOC meter from Blue Sea, it has a nice display but is limited with external controls. An OLED display is indeed very high contrast, I have Blue Sea Minis Voltage, Current (Shunt based house <> chassis) and temperature and indeed they are very high contrast. https://www.bluesea.com/products/1830/M2_DC_SoC_Monitor

https://www.bluesea.com/products/1830/M2_DC_SoC_Monitor

[booster]

Thanks to Marko for adding the link to the PWM vs MPPT discussion.


Another that we might as well have on here as well relates to how difficult it can be to determine how much power a given compressor frig is going to use, for comparison purposes when shopping.


Most important thing from this link is that the amp draw given is not an accurate way to have any real idea how much a frig is going to use in the real world. It will only tell you max possible use. The link also points out that you need to know what the freezer temp is set to run at as that is very important to some people, not to others, and keeping the freezer section warmer can significant increase the efficiency of the frig. Oddly, even though the frig will use less total energy with a warmer frig, the running amps are higher, which is part of why you shouldn't just use running amps as deciding factor. The link also shows the best way to decide, IMO, which is using tested energy use under fixed conditions. Isotherm gives you this information and a sample is shown.


http://www.classbforum.com/forums/f5...pecs-9459.html

[Kr4ftw3rk]

Thnx booster As a noob myself this is very valuable. Excellent information.