 | Home > Technical > How smart (or dumb) is the battery charging regime? |
 
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| BJH Member Since: 14 Apr 2022 Location: West Wales Posts: 17   
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I am going to add an auxilliary battery to my Defender and so first of all I want to get a detailed understanding of the way the standard setup works. My Defender is a 2009 Puma but I imagine that the situation is the same for practically all Defenders, although I'll be interested in the detail if somebody on the forum knows different.
THE WAY A LEAD ACID BATTERY SHOULD BE CHARGED The correct way to charge a lead acid battery is as follows: Apply a constant current (typically 0.3C), until the set voltage is reached (stage 1, 80% charged) Apply a constant voltage (absorbtion/topping phase) until the current drops close to zero (stage 2, 100% charged) Apply a lower constant voltage (maintenance phase) for as long as the battery remains 100% charged (stage 3) So, that much is clear to me. But what is not clear to me is how (or if!) this scheme is applied in a Defender or in cars more generally. HOW A TYPICAL VEHICLE ALTERNATOR WORKS So here is my current understanding of the alternator, which may be incomplete: The alternator produces an unregulated A.C. voltage which is subsequently rectified in a diode pack to convert it to an unregulated D.C. voltage. This is then fed to a regulator internally in the alternator such that the final output of the alternator is a constant D.C. voltage of approximately 14V. IS THERE SOMETHING IN THE MIDDLE? So we get to the core of my question. The constant voltage output of the alternator cannot apply the required charging sequence for a lead acid battery. For example, if you totally flattened your battery so that it only had 9V open-circuit voltage at its terminals, and then you got your vehicle started so that the alternator could charge it, the alternator would go ahead and regulate the voltage to 14V. The dead flat battery, once it got charging, would eventually consume quite a lot of current and effectively it would be subjected to a brutal charging sequence. Finally everything would get figured out, but the life of the battery would have been compromised. So this leads me to ask the question as to whether there is some middleware I don't know about. You can imagine putting a DC charger in between so that the charger is supplied the constant regulated voltage from the alternator and then the charger supplies the proper charging sequence to the battery. MY FINAL QUESTIONS 1. Is my understanding of the alternator correct? 2. If yes, then is there a charging middleware component between the alternator and the battery or do we just apply a dumb charging regime that will compromise the battery? 3. If there is middleware, where can I find it on my Defender? Thanks in advance for any responses. |
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15th Apr 2022 4:14pm |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17430 
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No, yes, no, and n/a.
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15th Apr 2022 4:39pm |
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| BJH Member Since: 14 Apr 2022 Location: West Wales Posts: 17
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Thank you for your reply. The reason for me asking all this is that I am planning the best way to fit an aux battery under the passenger seat. I have found out that there is nothing at all intelligent about the way the Defender (or any other typical vehicle for that matter) charges its lead-acid starter battery. It just has an alternator that regulates to ~14V and it applies that to the battery. Effectively charging it constant voltage. This probably doesn't do a great deal of harm when you're only starting the engine because the battery won't be depleted that much and so it'll be not far off the CV stage of the charging cycle anyway. But where it is an issue is if you flatten the battery. In this case CV charging is going to apply a pretty brutal charging cycle that will effectively damage the useful life of the battery. How often do you flatten your starter battery? Not often at all. But aux battery? all the time! That's how an aux battery is generally used. And so I can't just simply have a big switch that connects the aux battery to the starter battery when the engine is running or I'll end up with a leisure battery that doesn't last very long. Cheers. |
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16th Apr 2022 7:36am |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1081  
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I think you might be overthinking
Simple VSRs have worked fine with lead acid batteries for a long time. "How often do you flatten your starter battery? Not often at all. But aux battery? all the time! That's how an aux battery is generally used." Well yes and no. While deep cycle lead acids (gel) batteries seems to cope better with deep discharge, it's still better not to fully flatten them. I try to not go deeper than 12,0V no-load voltage which goes down to 11,8V when heater and frigde kick in. That's after 4 cold nights of camping. It's never been an issue to recharge while driving. Note, when the batteries are connected by the relay, there will be a compensating current between them. I measured it once with the aux at 12,2V and the starter battery at 11,8V. I connected a CTEK 10 to the starter battery and when the VSR enganged after 5 minutes, there was a current of about 30A between the batteries. But after a few seconds it went down to 6A. Charging voltage was 12,6V which was as much as the CTEK was able to maintain. The alternator should be stronger. fun fact: this 20 year old battery from my MINI survided the flooding of our barn 
 Click image to enlarge Last edited by macfrank on 16th Apr 2022 12:59pm. Edited 1 time in total |
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16th Apr 2022 9:19am |
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| MikeSel Member Since: 21 Mar 2022 Location: UK Posts: 10 
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I’ve been considering a second battery for camping/exploration. I’ve seen a number of dual solutions in parts catalogues, which I don’t currently have to hand. However they’re similar to this https://www.roamingtheoutback.com/2013/12/...tallation/
Which is likely the route I would take. |
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16th Apr 2022 11:25am |
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| Mo Murphy Member Since: 01 Jun 2008 Location: Letchworth Garden City, Herts Posts: 2241 
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Tdi tech.
150amp relay, an inline fuse, some wire. Simple and easily replaceable if I am "overlanding 🤣". Not as Gucci or as expensive as Blue Sea systems, I'm afraid but it's not a solution looking for a problem. If you are letting your main battery get so low that it won't start your vehicle without a connector switch without noticing or resolving the issue then you deserve to walk home. Just my tuppenceworth Mo The Land Rover 90 - Many are called, few are chosen. 50 Shades of Pennine Grey |
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16th Apr 2022 12:00pm |
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| BJH Member Since: 14 Apr 2022 Location: West Wales Posts: 17
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The point I was making here is that charging direct from the alternator only works for a battery which is not really discharged - like your starter battery under normal conditions where it is just used to briefly turnover the engine and then gets topped back up. The scenario where you have a discharged battery is quite different, and under these conditions it is not sensible to charge it direct from the alternator - certainly not regularly. And here I am not really referring to the starter battery but the aux battery which I intend to install. That battery is the one that will be regularly discharged.
Well, I'm thinking your solution is the one looking for a problem. I'm all for simple solutions but only if it's the simplest solution that satisfies the requirements. And when it comes to an aux battery, I don't think a bit of wire and a relay does that. It'll work, of course. But over time it will destroy the batteries and you'll get a mere fraction of the life from them that you would otherwise have got. |
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16th Apr 2022 10:45pm |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1081
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You only seem to acccept replys that fit your theory.
In practise the effect you worry about is not that relevant. Simple setups do work very well. Partially because you would design your system for a normal DoD of 50% anyway. Otherwise you would go Lithium. Partially because lead acid batts are no sissies. That‘s what I tried to illustrate with my photo if the drowned one that‘s still working fine. If you need a more sophisticated solution to satisfy your get a B2B charger. |
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17th Apr 2022 6:36am |
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| SteveG Member Since: 29 Nov 2011 Location: Norfolk Posts: 660 
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It’s quite simple really - go for some of the proven and simple setups suggested above or buy a DC to DC charger.
If you go for later, go for a tested make like Victron, Redarc and on the model to pick basically go for the lowest amperage model. You only need the higher rated versions if you’ve got a big capacity aux setup and/or you regularly have a limited time to replenish the aux battery system. |
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17th Apr 2022 6:48am |
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| Mo Murphy Member Since: 01 Jun 2008 Location: Letchworth Garden City, Herts Posts: 2241
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Interesting, but you've never actually tried it yourself, am I correct ? Your knowledge is from your Internet research ? The Land Rover 90 - Many are called, few are chosen. 50 Shades of Pennine Grey |
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17th Apr 2022 8:27am |
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| BJH Member Since: 14 Apr 2022 Location: West Wales Posts: 17
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I hope that's not the case  but I will take that constructive criticism on board.
I am definitely interested to hear from forum members who will say something like "here's my setup - a bit of wire and a big relay. Been using it for x time and no issues, no battery fatigue, no problems". That would definitely get my interest.
Agreed about the 50% DoD or go Lithium. But my understanding is a 50% discharged lead-acid directly connected to a CV source (the alternator) will be subjected to a brutal charging cycle and will have reduced service as a result. I accept that I haven't actually tried this myself in practice and that my claims are only based on the battery manufacturer's literature. I am actually considering Lithium due to the 50% rule. Because I want to run a fridge for at least 24 hours and it seems like I'd need a very big lead-acid battery to accommodate that (plus some other bits 'n' bobs I want to run off it) which then potentially will not fit under the passenger seat. Trouble is the obvious expense of a Lithium battery steers me back to a lead-acid 
I'm surprised that battery works at all after that! So if you're saying not only does it work, but it works well, then I am impressed.
Yeah so I've looked into this and actually you can pick up a system that will apply a proper charging profile, plus isolate the two batteries so that they charge separately (and therefore the applied profile suits both) for not that much money. But it's about the cost of replacing the battery so it's about weighing up whether my perception that the battery life will be reduced without it is worth guaranteeing the cost of another whole battery up front. Thanks all for your replies. |
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17th Apr 2022 8:48am |
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| BJH Member Since: 14 Apr 2022 Location: West Wales Posts: 17
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Thanks. Yes provided I can find a system that will run what I want off-grid for at least 24 hours then I don't think a fast charge will regularly be required. My trips are mainly just about having Father-Son time with my 4 year old so nothing too extravagant Cheers. |
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17th Apr 2022 8:53am |
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| Mo Murphy Member Since: 01 Jun 2008 Location: Letchworth Garden City, Herts Posts: 2241
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Here's my setup - a bit of wire and a big relay .... and a fuse. I've been using it on this 90 for 12 years, the same on the 90 previous to that for 8 years and the hybrid prior to that for 3 years. Its used for the fridge, scene lights, USB sockets, dash cam, heated mirrors, auxiliary fan and heated seats. The lead acid leisure batteries have a life of about 6 years which I think is reasonable and it'll run the fridge alone for about 3 days without moving. I've had one failure in that time, of a relay which I picked up on the dual voltmeter and that was replaced the same day. Click image to enlarge Click image to enlarge Hope that helps Mo The Land Rover 90 - Many are called, few are chosen. 50 Shades of Pennine Grey |
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17th Apr 2022 11:33am |
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| macfrank Member Since: 05 Nov 2015 Location: somewhere in the north Posts: 1081
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here's my setup http://kraxlr.de/makingof.html#electrics- a bit of wire and a big (voltage sensitive) relay. Been using it for about 200 times in 5 years and no issues, no noticeable battery fatigue, no problems.
Oh and if you factor in the cost: that aux was about 170€. It's said to last 750 full cycles at 50% DoD and 450 cycles at 80% I think.
That's a very big fridge  or a very inefficient one. Our Engel is spec'ed at 30W (2.5A @ 12.0V) but it's running for only 30 secs every 5 minutes or so. Of course that depends on how cold you want it to be and how hot it is outside. But even in the Namib at 36°C in the shade (if there had been any) we never had to turn it higher than "3" on it's 0 to 5 scale. It's usually on 2 while driving or when it's warm, 1.5 by day and at night we turn it down to 1.
Our Webasto heater takes 29W on high and 14W on low output. When camping in the cold we usually have it on 2-3 times a day for a few hours and sometimes also at night for an hour or so. Each time it's on high for about 15-20 minutes and on low for the rest of the time. With this usage, frigde + heater, we easily get two to three nights / 40-60 hrs and also 4 nights / 90hrs are possible when going down to 30% DoD.
I was surprised, too. It had sat in the water for two days. I was about to scrap it then and there, but instead reconditioned it with the CTEK and it actually worked well enough for the MINI. As the MINI sadly gathers dust and rust now, the battery is used as a small backup battery, charged by a solar panel once in a while. It still keeps it's charge quite well. |
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17th Apr 2022 11:50am |
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