Alternator charging and auxillary batte

[Boydie]

Okay, my alternator puts out 14.5 volts. The main 100 amp/hour battery has been relocated from the RH side of the car to the LH side, nearest to the alternator and starter motor, the 85amp deep cell auxiliary battery is in the original RH battery location.

Power from the alternator feeds the main battery via a 100amp manual reset Circuit Breaker. From the positive main battery connection power is supplied to the auxiliary deep cell battery via a RedArc relay, the supply being normal battery cable, rated at 150 amp. The alternator and chassis are earthed to the main battery, the auxiliary is earthed to the original battery location chassis earth point.

All connections have been checked and are clean and tight.

The 85 amp/hour deep cell auxiliary only feeds 3 cigarette style plug power points (seldom used apart from charging mobile phones etc.), It also supples two LED external camping lights - used sparingly at night- and the 47 litre, 450 watt fridge/freezer.

The problem I am experiencing is this, initially while the engine is running and driving along the fridge runs fine, however after the engine is off after the third day there is insufficient power in the deep cell to run the fridge for more than 3 hours during the night. After the seventh day even with the engine running there is barely sufficient voltage being supplied by the deep cell to run the fridge, even with it being set to cut out at 10.75 volts.

Obviously the 100 amp alternator even at 14.5 volts is insufficient to maintain charge to the main battery and re-charge the deep cell auxiliary battery.

Any ideas and or solutions ????

[Guest]

There is no reason (set up) why your system should not be working, and the 100 amp alternator should be plenty to supply both batteries I would therefore suggest you have a fault somewhere along the line.

Split charge not working correctly relay could be sticking etc

Short on the Aux battery

Worn battery on the Aux battery

Are the negatives wired back to the battery on the Aux battery or do they just go to the chassis VE-?

What voltage are you getting through the split charge?

What amperage are you getting through the split charge?

Can you take the Aux battery out and charge it on the bench and then carry out a discharge test?

Jason.

[Team Idris]

I'm no fan of the words 'split charge' and 'relay' together because connecting causes burning if the load is high. Once a relay has some age to it I don't think you are any worse off than the 0.7volt drop of a split charge diode pack. Plus, the diode pack stops power traveling back to the alternator if it shorts out. But I am probably being mean to the system as it isn't as harsh charging a deep cycle battery. My first test would be to measure the voltage drop across the main terminals of the relay when it is charging. That is a hell of a big load though from the fridge. (450 / 12).

probably worth adding that boats use a different regulator to push more current into deep cycle batteries. It is an external box-of-tricks with a specific charge curve.

[ejparrott]

Why is there a circuit breaker between the alternator and battery? That's not standard Land Rover wiring.

[Bowie69]

Is this the 'relay'?

http://www.redarc.com.au/products/product/dual-sense-smart-start-sbi-12v-100a/

[Maverik]

You have a current draw of 37.5amps from the fridge, you have a 85amp hour battery, basic calc says your batteries going to last 2.2hrs... If your pancaking the battery each time, its going to take a fair bit of driving to re-charge it up, especially as you're running the fridge at the same time, so you're taking ~40℅ of the alternator capacity just running that...?? I'd say your alternator and battery set up is a little undersized for the fridge?

[AMB]

I think that Maverik has part of the answer, but not all. If a "proper fridge" ie temperature controlled via built in thermostat then duty cycle will need to be taken into account and useable life may be extended by improving air circulation around the fridge. Even a low power fan could make a lot of difference to energy required. However, a lot of "fridges" are just cool boxes using a Peltier effect element. They take more or less constant current. I'm assuming from the wattage and the need for maintaining a decent beer temperature that it is thermostatically controlled.

Suspect that surface charge versus deep charge also comes into this. Energy is stored as a reversible chemical reaction. With a fully charged battery that chemical reaction is at the surface of the battery. Discharge it and the surface reaction has been "used up" and the slower deep reaction takes over. Net effect - higher internal impedance at lower states of charge.

I looked up some charge curves on deep cycle (leisure) lead acid batteries. Whilst charging, with 14.5 volts at the battery terminals terminal voltage, the associated battery charge can be somewhere between 85% and 110% of nominal depending on charge rate, with the 85% representing a C/5 rate and 110%, C/40. In your case C is 85A. Discharge your battery too low and it never reaches full capacity under charge. Because of the way that you run it, your main battery could well be running near the surface charge region and therefore hogging charge.

Does your alternator regulate at it's terminals or take a sense wire from the main battery?

[Boydie]

Wow, where to start, and thank you for you interest and comments.

Bowie, yes that's the RedArc relay that I have installed.

EJ the 100 amp CB is there as per the RedArc wiring diagram - the 100amp CB being there to protect the main battery from over-charging.

Jason. The alternator is earthed to the main battery and to the chassis and body. The auxiliary battery is earthed to the body and chassis. Volts through the RedArc relay is 14.5 volts, I will take an amp reading this morning and get back to you, with and without the ARB fridge running.

The Fridge/freezer is an ARB 47 litre unit. I am informed it draws a maximum of 450 watts but there is no power rating on the plate so I am unable to confirm this, it does seem a little too high. Electrically wise it has a compressor and condenser fan, from my observations while the tiny condenser fan runs continuously the compressor cuts in and out thermostatically, once down to temperature (it is normally set at 4 degrees) it hardly runs at all. It takes around 3 hours to get down to 4 degrees when loaded with pre-cooled product, meat, dairy, drinks etc. The built-in computer / control will run the fridge until the aux. battery is down to 10.5 volts.

I have 14.5 volts on the input and output connections on the RedArc relay and at the auxiliary positive terminal.

The main and the auxiliary batteries were both new in October of last year. 10/2014 both have been tested and are okay. The deep cell auxiliary was bench tested by Battery World (the supplier) by cycling it for discharge and recharge.

The only alternative that I can come up with is to install a third (second deep cell auxiliary) battery in the rear of the car. I used to have an Odyssey gel filled deep cycle mounted on a bracket on the rear chassis rail but removed it to reduce total weight, at this early stage of the "development" of "Brutus" I had two normal wet cell batteries up front, I removed the Odyssey and replaced it with the 85 amp/hour deep cell as you well know the standard wet cells don't like being continuously discharged, maybe I will have to go back to that 3 battery set up, there simply isn't room under the bonnet for bigger amperage auxiliary battery.

[cackshifter]

I'd maybe invest in an ammeter (at least temporarily, or use a clamp one) in the lead to the deep cycle battery - that'll give you an idea what is going in and out. Then try it all out stood outside your house. Also try charging it right up with a mains charger & again seeing how long it lasts with the fridge.I doubt the fridge will draw 37.5 amps for long - I expect the manufacturers are going for the worst case scenario of peak usage - if you think about it, it all eventually ends up as heat, so it has to dissipate that as heat plus whatever heat it has pumped, so it is going to be chucking out a lot of heat. Are you sure the charge isn't getting through something else? the 85 Ah is probably at a 10 hour rate, so you will probably not achieve anything like85Ah with a much faster discharge. But, every cloud... at least the battery isn't going to be cold!

[AMB]

Have just checked the specs on the ARB 47l. http://www.arb.com.au/products/fridges-camping-accessories/arb-fridge-freezers/

It reckons average power consumption of 0.87A/hr and 12V at 3C cabinet temp, external temp 26-31C tested over a 66 hour period.

The manual http://www.carid.com/images/arb/car-organizers/pdf/fridge-freezer-users-guide.pdf suggests a 15A fuse for 7A rated current, so roughly 10% duty cycle.

If these figures are correct and the battery is good, it would suggest that you are not charging the auxiliary - 85AH at 0.87A/hr is roughly 100 hours. Unless I have misunderstood your problem you are getting 75 hours. Rather a coincidence. Verify it by running the fridge directly from a fully charged auxiliary and see how long it runs.

[Boydie]

Thanks for that AMB,

I'm running the fridge as we speak, I started at 12.00 noon today so I will see how many hours I get out of the auxiliary which is fully charged and disconnected from the charging system --- I'll then reconnect it and see if and how long it takes the system to recharge the battery.

The fridge is full of cans of Julies sugar free soft drink (so that there is no temptation on my part to sample said product) - these came straight out of the fridge in my man shed and were already chilled. This is a win/win as I re-stocked it with manly amber liquid .

The fridge reached set point of 4 degrees after 45 minutes, compressor off.

[AMB]

As implied above, don't rely on voltage to tell you when the battery is fully charged. Leisure batteries have differeent charge/discharge characteristics to standard automotive batteries and it's not just the depth of discharge. Automotive batteries try to maximise plate surface area which helps to give the high current capability - a lot of dependence on surface charge. Leisure batteries have more solid plates and so depend more on deep charge. Try and recharge at too high a rate and you'll never charge to full capacity. The effect is even obvious on standard auto batteries. My battery charger has auto voltage cut-off and two charging current levels (equates to C/8 and C/20). The C/8 rate only takes me to 83% of capacity, C/20 gets me another 15-17%.

Richard Perez has written quite a lot on the subject at a laymans level http://www.zetatalk4.com/docs/Batteries/FAQ/State_Of_Charge_Ver_Voltage_2004+.pdf

[Boydie]

Right, 8.00am this morning the fridge switched off, battery voltage being 10.5 volts so its ran the fridge for 44 hours.

The next step is I guess to see how long it takes to re-charge back up to maximum amps/voltage. I'm toying with the idea of installing a relay to switch the charge from the alternator direct to the auxiliary rather than relying on the current system with the RedArc which charges the main first then and only then allows any "excess" to go the auxiliary. If I installed a relay, power off it could supply the system as is, (normally closed contacts) and power on (normally open contacts) it would supply all charge to the auxiliary. When switched off it would continue to charge the system via the RedArc as normal.

Any thoughts on this idea ?

[AMB]

The RedArc system as described ought to work. If I've guessed at the right system and read the specs correctly, it should isolate when the main battery is 12.7v or less and connect both batteries together once the voltage exceeds 13.2v. So, perhaps an initial delay in charging the auxiliary after starting, until the main battery voltage reaches 13.2 v

Need to perform a logical check through the system. Where does the RedArc pick up it's earth? Is there a voltage between RedArc earth and Main earth? and Aux earth? between Aux and main earth?

[Boydie]

The RedArc is earthed to the same chassis lug as the auxiliary battery. No voltage between the unit and it's earth. No voltage leakage between the aux. battery and earth. The problem is (I am told by Battery World who sold me the deep cell aux. battery is it will need 14 volts to re-charge it, and that the "trickle" charge of 12.7 to 13.5 volts is insufficient to recharge it when it is a fully discharged state, i.e.. 10 volts or less.

The main battery is fully charged, - measured at 14.5 volts (static) the voltage supply to the RedArc is 13.2 volts. the same voltage is at the aux. battery, so maybe I have to somehow increase the supply voltage ???

[cackshifter]

Re your relay idea to switch not quite so simple as it seems as alternators can die if even momentarily disconnected from a battery. How about a second alternator? But before that, are you sure you are getting full voltage at the battery, you should see around 14.2v to 14.5v with the engine at a fast idle. Sometimes one diode can fail in an alternator and you don't quite get the full voltage. But once the volts rise to 13.2 (which should be PDQ) the RedArc should turn on and then both batteries will be getting charged - maybe that drops the volts again and the RedArc turns off again. One other question, if the main batt is at 14.5V, why is the RedArc supply only 13.2? I think if that stays at 13.2 that is the nub of your problem.

[Maverik]

Re your relay idea to switch not quite so simple as it seems as alternators can die if even momentarily disconnected from a battery

Never heard that before... What's the rational?

[AMB]

Boydie - Which RedArc version do you have? Looks like some models have an override wire which may help. However, like cackshifter I'm a little puzzled.

Why is there 1.3v drop between the main battery and the RedArc input? Volts drop implies current and/or resistance. If you have 14.5v at the main battery, standard (150A?) wiring to the RedArc and 1.3v drop across that wire then that implies a fair current going somewhere - or a little current across a big resistance. 13.2v at the RedArc may mean that it has not switched on. Once switched on it ought to hold, even if the main battery feed drops to just above 12.7v

You could try artificially increasing the voltage at the RedArc input by routing the alternator positive first to the RedArc, then to the main battery. Take the RedArc earth to the main battery earth. Test by replacing the Aux battery with a bulb across the Redarc output to see when it switches?

At 13.2 v the aux battery is unlikely to reach full charge, but should still charge to >60% or so.

Maverik - alternators have an inductive source impedance. If you interrupt current in an inductor you get voltage flyback (as in coil ignition). High(er) voltages can blow the diode stack, especially on older alternators - cheap, low voltage rating diodes used. Though I haven't heard of one blowing for some years now.

[Maverik]

Some interesting reading with regard to voltage flyback and as suggested its more common mode of failure for older style alternators, but more of an issue with sensitive electrics attached to said alternator. But I'd say you could rule out flyback with the action of just switching alternator load, or you would see more alternator failures on vehicles with simple split charge relay systems surly?...

[AMB]

As I said - haven't seen a blown alternator on switching for some years. They used to be fitted with rectifier stacks rated at only 16v and could be tender as a result. They usually failed before the spike hit the radio - not much else in the way of electronics in those days. I had access to high power diodes and a decent press through my job and have repaired a few in my time

[Les Brock]

The alternator issue is why on a FIA battery cut off switch which is deigned to be used with the engine running has two sets of contacts one pair open and one close when you operate the switch.

the set that close dumps the spike voltage to ground via a resistor so it does not potentially pop the alternator, by seeing no battery resistance

[Boydie]

Right, at the alternator I have 14.56 volts (purchased a new Scope multimeter) The main battery, when disconnected has also 14.5 volts. When connected, and the engine on 780rmp, idle speed, I am getting 14.5 volts charge to the main battery, which is now reading 13.5 volts, at the input and output to the SBI12D RedArc unit I have 13.5 volts. I have run the auxiliary battery down to just under 10 volts. This is according to Battery World insufficient voltage to recharge the deep cell 80 amp/hour auxiliary battery - it needs at least 14 volts. Any ideas ????

I'm seriously thinking of having the initial charge go to the auxiliary and then through the SBI12D to the main although this goes against the grain

[Bowie69]

Measure at each point between alternator and aux battery, and note voltages, it is quite common to lose 0.2V across a connection/length of cable, this way you can diagnose where the voltage is disappearing....

This is assuming you haven't just done that Factory looms used to (on the RRC anyways) have a bobbin on the firewall which took loads of connections to places, not a good way, as got corroded very easily ....

[AMB]

Bowie69 is obviously thinking along the same lines as me.

You are apparently losing 1 volt across what should be a very short length of high current rated cable between the main battery and the RedArc. To put that into context I measured just over 1.4 volts drop between battery and headlamps for a nominal current of 10A, a comparatively lightweight cable and a longish run. If you had a similar order of cable drop/metre between alternator and main battery it would be unlikely to charge.

I haven't checked the formula for accuracy, but RedArc suggest that voltage drop across a cable equals length (in metres) times current (in Amps) times 0.017 divided by Cross sectional area of copper in square millimetres. That drop will increase with temperature, including self heating due to current carried. Using that formula, assuming 1m of 8 square mm cross section cable between Main Battery and RedArc input at 60A would give a voltage drop of 0.13volts plus connector drop.

Why the voltage drop? Measurement error? - are you using the same reference earth for all measurements? The cables may be OK, ditto the earth connection, but what about the crimped connectors? (Note, if done properly crimped connections should be lower impedance than soldered).

I think that your best option may be to run the alternator connection directly to the RedArc input, then to the main battery. The RedArc needs to be mounted as close to the main battery as possible, using heavy duty cable and the same earth/negative terminal as the main battery. That arrangement should ensure that the RedArc accurately measures the main battery voltage and therefore gives maximum charging time for the auxiliary. Routing the alternator feed to the main battery via the RedArc input terminal should (very slightly) increase the voltage available to charge the auxiliary.

If I've interpreted the RedArc spec correctly it has 0.5 volts of hysteresis ie needs to exceed 13.2 volts for 5 seconds in order to switch the auxiliary battery into circuit, then maintains connection until the main battery drops below 12.7 volts. That does imply a short period of battery energy sharing after the engine has shut off over and above the stated 10s switch off delay. RedArc have obviously seen some of the problems you've experienced, in that they recommend that "start and auxiliary batteries be of similar chemical characteristics when charging via the Smart Start® SBI".

When using this in anger (ie when the fridge cuts out after the third day) how many hours charging time do you estimate that the system may have?

[Boydie]

I think the problem may be the RedArc, when charging it back up to full capacity today with a solid state 40 amp charger the RedArc switched on when the auxiliary reached 13.5 volts, - effectively causing the charger to recharge both batteries, so I'm thinking it may be defective as from what I've read it should only switch when sensing the main battery is above 12.7 volts. The alternative is, as you said to relocate the RedArc close to the alternator.