Maverik Posted February 28, 2014 Share Posted February 28, 2014 Hi Folks, appologies in advance if this is a stupid question, but got a spare minute to think about a split charge system I'm about to instal on a truck. Been meaning to get my head straight about this for a while. So the question, a "conventional" split charge system, that uses for example a 12v 100amp relay. Is this type of system a purely "automatic" isolation system i.e. all it does is close a parallel circuit to the 2nd battery to allow it to be charged when the engine is on i.e. alternator operating normally. As soon as the engine is stopped and the alternator is turned off, the relay opens thus disconnecting the two parallel batteries...? If its more than this then someone really has to explain the details, or there's some secret black magic involved. There are so many miss leading threads on this topic splashed about the whole web... I know you can get VSR modules and the like and thats another topic for me to explore. And to start with there is a very interesting article on it here http://www.adverc.co.uk/batterymanagement/split-charging-systems Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted February 28, 2014 Share Posted February 28, 2014 That's pretty much it, Mav. Quote Link to comment Share on other sites More sharing options...
Les Brock Posted February 28, 2014 Share Posted February 28, 2014 What Mo said Quote Link to comment Share on other sites More sharing options...
miketomcat Posted February 28, 2014 Share Posted February 28, 2014 If you use the system you describe you tend to drain your batterys down to the same level before charging them not good with short trips. If however you use a vsr (fitted standard to some early 90/110 and range rovers) to trigger your relay/solenoid (basically what the expensive systems are) it gives the first battery to reach full charge before draining it by linking to the other one (less overall drain). Then there's split charge diodes but that's different again. Mike Quote Link to comment Share on other sites More sharing options...
AMB Posted February 28, 2014 Share Posted February 28, 2014 The basic split charge system using a relay operates as you suggest - when the engine is on it energises the relay, closing the connection between auxiliary and main battery. When the engine stops, the relay is unergised and disconnects the two - allowing you to use the auxilairy battery without risking flattenning the main (starting) battery. As Mike says, disadvantage is that, when you try and start the engine with auxiliary discharged, the main battery will face a heavy surge as it tries to charge up the auxiliary - effectively trying to share the charge between the two batteries. A battery isolator between relay and batteries is a good idea if this is going to be a problem. Or put a switch in the earth return of the relay coil to isolate the second battery if necessary. In my opinion voltage sensing relays don't offer much advantage and can be unreliable. Determining when a battery is charged using voltage alone (when being charged by the engine/alternator combination) can be tricky. It will be temperature dependent, battery condition dependent and load dependent. I've yet to see a VSR system which will cope efficiently with all of these. Diode split systems provide isolation between the batteries, but have an associated voltage drop which ideally requires an auxiliary alternator voltage regulator to compensate. Consideration also needs to be given to switch-over between battery functions unless you fancy swapping batteries over in the cold and wet when your main fails. The most reliable system that I've experienced was in a yacht - two batteries with manual switching to allow parallel or single operation and switch battery function. Quote Link to comment Share on other sites More sharing options...
vulcan bomber Posted February 28, 2014 Share Posted February 28, 2014 A simple 30amp relay is all you need.. Or do it properly and fit a 2nd alt. Quote Link to comment Share on other sites More sharing options...
Maverik Posted February 28, 2014 Author Share Posted February 28, 2014 Thanks folks, some good comments which pretty much have confirmed my thinking so far. - and a perticular interesting note about the Aux to Main surge problem. Also now starting to understand why the type of relay etc. and in operation use has an effect on what system should be employed... Quote Link to comment Share on other sites More sharing options...
miketomcat Posted February 28, 2014 Share Posted February 28, 2014 A simple 30amp relay is all you need.. Or do it properly and fit a 2nd alt.The only problem is if you have a winch you can get a situation where your trying to draw 300 amps across a 30 amp relay I've seen an Albright melted because of this problem.Most yachts now use either a split charge diode, second alt or as I have for the ibex a duel sensing vsr which I think drops out if the voltage drops more than the alt can put in (must check it no prizes for where I got it from). Mike Quote Link to comment Share on other sites More sharing options...
vulcan bomber Posted February 28, 2014 Share Posted February 28, 2014 The only problem is if you have a winch you can get a situation where your trying to draw 300 amps across a 30 amp relay I've seen an Albright melted because of this problem. Most yachts now use either a split charge diode, second alt or as I have for the ibex a duel sensing vsr which I think drops out if the voltage drops more than the alt can put in (must check it no prizes for where I got it from). Mike Fit a big isolator accross the battery positives, giving the added bonus of being able to jump the main battery off the Aux battery if needs be. Quote Link to comment Share on other sites More sharing options...
SORNagain Posted February 28, 2014 Share Posted February 28, 2014 In my experience the battery surge problem isn't really a problem in practice. Even with a depleted battery although the initial charge current can be quite high, it quickly settles down to 20A or so, and then tapers down from there. At the point where the alternator can cope with both loads, neither battery is being depleted. This generally happens so quickly it's not worth worrying about. I have about 200Ah of auxilliary battery in my truck, a 90A alternator, normal split charge relay, and current meters on both the starter and the auxiliary batteries. I have never seen the starter battery swing negative when the relay clicks in, regardless of the state of the auxilliary. It does swing negative if I have the headlights on and my wife puts a couple of slices of bread in the toaster, but that's a different story... I second the isolator across the positives for jump starting...and it makes for an easy place to mount your relay. Quote Link to comment Share on other sites More sharing options...
AMB Posted February 28, 2014 Share Posted February 28, 2014 Sorry to disagree SORNagain. Although you may not have experienced a problem in practice, the higher charge battery will always be depleted when connected across to a discharged battery and the alternator will always disproportionately charge the discharged battery until the charges equalise. Whether you see discharge from the main battery or not will depend on where you wire your ammeters relative to the relay and the state of the batteries. Quote Link to comment Share on other sites More sharing options...
SORNagain Posted February 28, 2014 Share Posted February 28, 2014 I don't think so. Provided the alternator can provide enough current, neither battery will be discharging. Quote Link to comment Share on other sites More sharing options...
AMB Posted February 28, 2014 Share Posted February 28, 2014 You cannot escape battery charge equalisation with a simple relay system. Simple physics. Battery at full charge and 20C = 13.6v. More than 20% discharged and 20C <, = 12.0v. Connect the two in parallel and the voltage across them must be the same. The only way that can happen is if the higher voltage battery discharges into the lower (aux) battery. The alternator charging current will split between the two batteries with the majority of current assisting to charge the discharged battery Internal resistance of lead acid goes up with discharge by 40-50% and also changes with temperature and age/condition. At low/medium currents, internal impedance fully charged is roughly 10 mOhm, so connect the two in parallel with a voltage differential of 1.6v and 25 mOhm = 64A peak, less if batteries not in top condition. Allow perhaps 10mOhm for the relay and you still have 46A Nominal alternator output of say 70A means that it should cope with normal demands, but not a lot in hand and you won't start to charge the main battery until the voltage across the two equalises. Quote Link to comment Share on other sites More sharing options...
mmgemini Posted March 1, 2014 Share Posted March 1, 2014 Isn't all this theory marvelous ? I use a simple Lucas SRB630 split charge relay. Well so for four trips to southern Africa and no problems. It's BUSH repairable in as much it can be swapped with an ordinary relay. It's been on my Defender since 2003. Cost, under twenty quid at the time. Quote Link to comment Share on other sites More sharing options...
AMB Posted March 1, 2014 Share Posted March 1, 2014 Mike, In my case theory is backed by practice, albeit mainly sailing and some work with trials vehicles and jury rigged systems - where the system is used in earnest. It's also a similar situation as jump starting a second car. Would you do that without your engine running first? The only points that I'm trying to make are that the simple relay system has disadvantages and consequently you cannot entirely rely on the method to safeguard engine starting from the main battery. There are also potential hidden issues such as reduced battery life. Obviously, medium use of the auxiliary battery (no deep discharge), reasonable time for both batteries to re-charge whilst the engine is running etc and you are unlikely to experience many problems in practice. The theory does assume good electrical installation - appropriate rating of cabling and relay, crimped connectors or crimped and soldered rather than just soldered etc. The poorer the installation, the less of a surge issue. I'd suggest that put a switch in the feed/return of the relay coil so that you can manually isolate the second battery. The gold plated solution is a second alternator with the ability to switch either battery as main or aux, next down is split diode (with auxiliary regulator and remote sense). When it comes to VSRs, you need to know what you are buying. Too easy to buy something with no practical advantages. Quote Link to comment Share on other sites More sharing options...
mmgemini Posted March 1, 2014 Share Posted March 1, 2014 Thank you for pointing out that I have never used my vehicle to travel. Quote Link to comment Share on other sites More sharing options...
AMB Posted March 1, 2014 Share Posted March 1, 2014 Thank you for pointing out that I have never used my vehicle to travel. Not my intention, or what I said. Apologies if you read more into it. Not having a go at you, just making the point that there was practical experience as well as theory in my response. It was part of my job. You can get away with a simple relay, but bear in mind the disadvantages and don't think that you are always going to have the benefit of a fully charged main battery, despite heavily using the auxiliary. Quote Link to comment Share on other sites More sharing options...
Les Brock Posted March 1, 2014 Share Posted March 1, 2014 Mine, simplistic version - Twin alernators, twin batteries, with a master switch to link batteries should either one alternator fail, job done Quote Link to comment Share on other sites More sharing options...
SORNagain Posted March 1, 2014 Share Posted March 1, 2014 Uh-oh, can open worms everywhere! 'Simple physics' would suggest that if you have an alternator capable of supplying the required current, both batteries will be charging happily. My original comment was that the duration for which the alternator can't cope is generally so short it's not worth worrying about, as I illustrated in my set up. I think that unless Mav has hundreds and hundreds of Ah in the back of his truck a normal relay will be fine. I appreciate that is not the perfect solution, but as land rover owners we have to be pragmatic! Quote Link to comment Share on other sites More sharing options...
mmgemini Posted March 1, 2014 Share Posted March 1, 2014 Not my intention, or what I said. Apologies if you read more into it. Not having a go at you, just making the point that there was practical experience as well as theory in my response. It was part of my job. You can get away with a simple relay, but bear in mind the disadvantages and don't think that you are always going to have the benefit of a fully charged main battery, despite heavily using the auxiliary. LOL Not read as that.... As I built my truck in 2003/ 4 things have changed. I would do it differently now. By the way. Why do you need an engine on a sail boat ? Quote Link to comment Share on other sites More sharing options...
miketomcat Posted March 1, 2014 Share Posted March 1, 2014 Trust me mariners get really really upset if you sail into your berth let alone doing it with the spinnaker up. Don't ask me how I know........ Mike Quote Link to comment Share on other sites More sharing options...
AMB Posted March 1, 2014 Share Posted March 1, 2014 At risk of tipping the can over and letting the worms breed .... one of the batteries will be discharging as the other charges, until they equalise. Recovery time is a different issue. Take a simple example. If you have two identical batteries, one fully charged and the second discharged, then connect them together. The fully charged will charge the other. Though the alternator will have some effect at this time, it will be smaller than battery to battery because of the different source impedances and longer cabling. If the batteries are of the same type and capacity they will try to equalise at an end point, approx 45-47% of maximum charge. If the batteries are in reasonable condition you will probably not have a noticeable issue. However, the peak currents involved will tend to pit relay contacts if too low a rating fitted. There may also be an effect on overall battery life. Assuming, battery capacity of 70Ah apiece, alternator rating of 70A and charging efficiency of 90%, it will take at least 1.25 hours of charging before the main battery is at full charge. In practice well over 1.25 hours because the alternator is only rated at 70A and is not capable of supplying this under all conditions, the calculation also neglects engine cranking current and other electrical demands on the system in the meantime. Consequently, the relay solution is generally better suited to expedition use than to everyday travel because of the increased amount of time that the alternator is available Re sailboat. The owner likes ice in his G&T, I prefer my beer a little cooler than it would be if just left in the cabin and both of us have bad memories of mains hook-ups in marinas! Have also had to sail into a berth a few times and it's not an experience I'd like to repeat on a regular basis, though I did learn a few words .... Quote Link to comment Share on other sites More sharing options...
vulcan bomber Posted March 1, 2014 Share Posted March 1, 2014 At risk of tipping the can over and letting the worms breed .... one of the batteries will be discharging as the other charges, until they equalise. Recovery time is a different issue. Take a simple example. If you have two identical batteries, one fully charged and the second discharged, then connect them together. The fully charged will charge the other. Though the alternator will have some effect at this time, it will be smaller than battery to battery because of the different source impedances and longer cabling. If the batteries are of the same type and capacity they will try to equalise at an end point, approx 45-47% of maximum charge. If the batteries are in reasonable condition you will probably not have a noticeable issue. However, the peak currents involved will tend to pit relay contacts if too low a rating fitted. There may also be an effect on overall battery life. Assuming, battery capacity of 70Ah apiece, alternator rating of 70A and charging efficiency of 90%, it will take at least 1.25 hours of charging before the main battery is at full charge. In practice well over 1.25 hours because the alternator is only rated at 70A and is not capable of supplying this under all conditions, the calculation also neglects engine cranking current and other electrical demands on the system in the meantime. Consequently, the relay solution is generally better suited to expedition use than to everyday travel because of the increased amount of time that the alternator is available Re sailboat. The owner likes ice in his G&T, I prefer my beer a little cooler than it would be if just left in the cabin and both of us have bad memories of mains hook-ups in marinas! Have also had to sail into a berth a few times and it's not an experience I'd like to repeat on a regular basis, though I did learn a few words .... After a couple of mins charging a battery wont accept much more than 8Amps, the only way you can alter that is to increace the voltage from the alt... Quote Link to comment Share on other sites More sharing options...
AMB Posted March 1, 2014 Share Posted March 1, 2014 After a couple of mins charging a battery wont accept much more than 8Amps, the only way you can alter that is to increace the voltage from the alt... Having apparently opened a can of worms and then spread them about liberally, I wasn't going to raise that issue directly, but you're right. You can't buck physics. It's horses for courses and you need to understand the advantages and disadvantages of each approach before selecting for your intended use. Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted March 1, 2014 Share Posted March 1, 2014 Thanks for that AMB, very interesting and enlightening. Sounds like Les's setup is the simplest and most versatile with 2 alternators and a big switch Mo Quote Link to comment Share on other sites More sharing options...
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