Battery management is about knowing how much capacity your battery has left in real time - similar to how a fuel gauge works on a fuel tank. It is very useful and informative to know just how much charge is flowing into and out of your battery, and how much power is left in the battery.
To do this you need a gadget that measures all the current going into and out of a battery and uses these measurements to calculate the state of charge of the battery. Such a gadget is called a battery management system. Another name is a battery monitor.
Typical battery monitor
A battery manager consists of 2 main components - the sensor, and the display. The sensor measures the current and the display gives you the information.
Technical alert! Don't worry if you don't understand this paragraph! The sensor device that enables these measurements and calculations to be made is called a shunt. In simple terms, a shunt is a lump of copper through which all current in and out of the battery must pass. The shunt is actually a carefully designed piece of copper that has a precise resistance which can be measured as a tiny voltage - and these tiny voltage are directly proportional to the current. By sampling these voltages many times a second, a simple computer on a chip can calculate and store these values, and if it already knows the capacity of the battery, then it can calculate how much power is left in the battery. Not only that, it can calculate how much longer the battery will last under the current load, and in a charge situation, how much longer the battery will take to get fully charged for a given amount of charge.
For the non technical, all you need to know is that a shunt measures the power in and out of your battery.
In terms of our beloved Classic Hymers, they actually do include a shunt, which is buried inside the back of the fuse box, but this was long before cheap chip computers were available, so all that was possible back then is to use the shunt to drive a meter - the famous "Strom" meter on the panel, Strom being german for power. If the battery is receiving charge, the the meter would deflect into the green, if the battery is giving power, then it goes the other way into the red. The Strom meter is also calibrated in amps - plus or minus 25 or 30, but because of the technology of the time and the tolerances of the shunt, this is only a rough indication.
But nowadays you can do so much better!
So how do you fit a battery management system (BM) to a Classic? It is a lot easier than you would think.
As already mentioned a BM consists of 2 main components - the shunt, and the display. The shunt goes on or near the battery, and the display goes wherever is convenient.
Although the original Hymer shunt is fitted in the positive side of the battery system, and buried inside the fusebox, the modern convention is to fit a shunt on the negative side of the battery, and the easiest way to do that is to simply fit it directly to the negative post of the battery. You just take all the connections that were on the neg of the battery, attach them to one end of the shunt, and then attach the other end of the shunt to the battery neg. It's that easy. Some shunt designs are now so clever that they attach directly, and can be fitted in a couple of minutes. This means that it is easy to fit a shunt to a Classic Hymer.
But nowadays you can do so much better!
So how do you fit a battery management system (BM) to a Classic? It is a lot easier than you would think.
As already mentioned a BM consists of 2 main components - the shunt, and the display. The shunt goes on or near the battery, and the display goes wherever is convenient.
Although the original Hymer shunt is fitted in the positive side of the battery system, and buried inside the fusebox, the modern convention is to fit a shunt on the negative side of the battery, and the easiest way to do that is to simply fit it directly to the negative post of the battery. You just take all the connections that were on the neg of the battery, attach them to one end of the shunt, and then attach the other end of the shunt to the battery neg. It's that easy. Some shunt designs are now so clever that they attach directly, and can be fitted in a couple of minutes. This means that it is easy to fit a shunt to a Classic Hymer.
The other component is a display, and this is also much easier to install than in the past because it doesn't need any additional wiring between it and the sensor because it communicates to the sensor wirelessly - via radio. All it needs is a 12v supply from the nearest convenient point.
The display contains a small computer chip which does all the calculations - all you have to do is to program it with the total capacity of your battery in amp hours (ah). It needs a few charge cycles to settle down and make its calculations, then after that it is quite accurate.
Here are 2 typical displays (there are many others) - you get an accurate display of system voltage in this case 12.29v. Below that is the number of amps - in this case the battery is discharging 2amps. If it was being charged, the number would be preceded by a + sign. To the left there is a simple battery meter which is showing less than half full - the exact figure is shown below 33%. There are other numbers that represent amp hours and watt hours, as well as a clock and battery temperature gauge.
Recent developments have seen a whole new raft of products come on to the market. One of these is Bluetooth. You can now use your phone or tablet to display the information, and the data is sent from a shunt that has Bluetooth - also known, strangely enough, as a Bluetooth shunt! These are very popular, but you should think about it carefully before you buy one. The main thing is that in order to see the state of the battery, you have to get out your phone or tablet and launch the app - not always convenient. So you might prefer to have a permanent display, so all you have to do is glance at it, in which case you get a BM that has a dedicated display. However some folk are having the best of both worlds by dedicating an old phone, or cheap tablet, to be a permanent van display. This can also be used for many other things - limited only by your imagination.
Once installed and working, you then have access to a whole new world of information about your battery system. There is quite a lot of information on the display, but the three most important values are voltage, amps and a percentage bar graph that tells you the state of your battery - similar to a fuel gauge.
Here are some examples.
Every time you switch something on in the van, the display changes and tells you exactly how much power that item is using. You can then, if necessary, modify your behaviour to suit how much power you have left. For example, if you switch on your TV or computer, you will be able to see just how many hours of use you will be able to get from the power available.
If you have solar panels then you can tell how much solar power is going into your battery, less any power that you are using. At a glance you can tell how long it will take for the battery to reach 100%. This is particularly useful when there is not much sunshine about. Many solar controllers have power displays already, but they only tell you how much power your panels are generating - they don't tell you the overall state of charge of your battery - ie how full it is.
While driving, the display will tell you how much charge is going into the battery from the van engine. You can then work out how much driving you need to do before your battery is fully charged.
When on hookup, you can tell how much power your mains charger is delivering.
Power management in a van is important, especially if you spend a lot of time in the van, and more especially if you use quite a lot of power - using a laptop or watching TV are usually the main consumers of battery power. It is very useful to be able to tell at a glance how much power you have left, and also what you need to do in order to get power back into the battery - either from solar, engine or hookup. It is particularly useful with solar, because after a few weeks or months of use, you will get a feeling for just how much power you can expect from solar in any given weather situation. Solar gives you much less power in winter and it is very useful to know exactly how much you can expect from solar, and modify your plans accordingly.
Another great advantage of a battery monitor is that it enables you to avoid abusing your battery, which can shorten its life. Most people know that a lead acid battery should not be discharged below 12v, which is roughly half of its rated capacity. This means in theory that a 100ah battery should give you 50ah of usable power before it drops below 12v. In practice this is almost impossible know without a battery monitor. With an accurate battery monitor you can see the voltage and capacity of your battery on the display, and you can tell exactly how much power you have left. This enables you to make decisions about reducing your power usage to get the best out of what you have left, and ultimately, to switch everything off and go to bed.
Once you get used to the information at your disposal you can then plan your activity accordingly. If you need to work on your computer for a few hours, or watch a movie tonight, you can tell at a glance whether you have enough power to do so. If it has been a cloudy weekend, wildcamping, and you want to stay put, then you will know how much battery you have left, and ration out the power - ie read a book instead of watch TV.
It may sound a bit daft, but 12v battery systems are not simple mechanical devices that are either on or off, with a fixed capacity. They have character! Battery systems perform differently according to conditions - how much charge, how fast, the temperature ... and they also change as they age. Only with a battery monitor can you get intimate with your battery!
It may sound a bit daft, but 12v battery systems are not simple mechanical devices that are either on or off, with a fixed capacity. They have character! Battery systems perform differently according to conditions - how much charge, how fast, the temperature ... and they also change as they age. Only with a battery monitor can you get intimate with your battery!
So who would benefit most from a battery monitor? The simple answer is - anybody to who battery power is or has been an issue. If you have ever run out of power unexpectedly or wondered how much power you have left, then you will benefit. The only people who don't need a battery monitor are those who always have enough power for their activities, and never run out. Many people don't run TVs or laptops and only need power for lighting and reading. If you always use EHU then you don't - if you always wildcamp - you do!
Other options - more technical stuff that you can skip!
There is another way of measuring current that doesn't use a copper shunt. This involves using a sensor known as a "Hall effect" sensor, named after the man who discovered it. This works by sensing the magnetic field generated by DC current flowing in a wire. It is the same effect as used in those test multimeters you can buy known as "DC Clamp meters". A hall effect sensor works by having the negative cable passing through it.
These have only become available recently - mostly from China. I have used one, and found it to be very good, but slightly more temperamental than a copper shunt. They also have to be located carefully, as they are easily damaged by knocks etc - we want things to last for years not months.
Another recent development is that fixed displays can now be fed from the shunt by radio - which means no wiring. This makes installation easier. You may ask what is the difference between this and a Bluetooth shunt? Good question - the answer is not much - one used a dedicated display but with no wires - the other is a phone app.
There is another way of measuring current that doesn't use a copper shunt. This involves using a sensor known as a "Hall effect" sensor, named after the man who discovered it. This works by sensing the magnetic field generated by DC current flowing in a wire. It is the same effect as used in those test multimeters you can buy known as "DC Clamp meters". A hall effect sensor works by having the negative cable passing through it.
These have only become available recently - mostly from China. I have used one, and found it to be very good, but slightly more temperamental than a copper shunt. They also have to be located carefully, as they are easily damaged by knocks etc - we want things to last for years not months.
Another recent development is that fixed displays can now be fed from the shunt by radio - which means no wiring. This makes installation easier. You may ask what is the difference between this and a Bluetooth shunt? Good question - the answer is not much - one used a dedicated display but with no wires - the other is a phone app.
A typical Hall sensor - the negative wire goes through the hole
So what should you buy?
Here is a list of the battery management systems that I know about
Victron - Victron now do a SmartShunt - a snazzy bluetooth shunt that drives their phone app - it is reasonably priced.
Victron SmartShunt and the App
Victron BMV - you get the full kit with Victron
Nasa BM1, BM1 Compact, and the NASA shunt
NASA have also just brought out a Bluetooth shunt and app.
The new NASA Bluetooth unit
BMPRO - BMPRO are an Australian outfit that produce quality gear - and their latest product is called the Batterycheck. This is a very simple design that blots directly to the battery, and displays on a phone app. They also have some other really fab products, and I believe they are just starting to get UK distribution.
Lovely product ... just bolt one side to the battery, and everything else to the other side! Google "BMPRO" for their other stuff.
China - China has a bewildering array of products available. The reason for this is that BMs are not just for motorhomes. They are used in electric bikes and electric vehicles and in off grid power setups - the principles are all the same - counting amps and calculating capacities. These are available on Ebay, Amazon and Ali Express. I can't possibly review them all. Some use shunts, some use Hall effect sensors. Most cost between $20 and $50 - that's US dollars. Anything cheaper will be no good. The problem is the specifications - the english is always bad. You want something that has a shunt, is for measuring DC and looks like these pictures here. If in doubt get in touch with me. Not all these products are crap - if money is tight, then no reason why not. At these prices you can just play! Most are unbranded, but Juntek seems to have a decent reputation.
Here is a selection of Chinese products - these are the main ones - my personal favourite, which I have used, is the bottom one, often labelled Juntek - available in several versions - including copper shunt, Hall effect sensor, hard wired, non wired (radio).
Finally - a note about Lithium. Lithium batteries are not like normal lead acid batteries. A lithium battery ALWAYS has an electronic battery management system, also called a BM or a BMS. Do not get these confused with what I am talking about here. This article is about BM systems for lead acid batteries.
If you want more information on this subject message me, or contact me on "Classic Hymers Technical" Facebook group.
If you want more information on this subject message me, or contact me on "Classic Hymers Technical" Facebook group.
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