Twin Charge Kit

Hi Ray
I have a Arrid BCTC25 Twin Charge which I am happy with.
I bought mine mail order through "The 12 Volt Shop" in Perth.
You could try them for info. on your model.
Their phone no is 08-94581212 or www.12volt.com.au
Or you could try "Arrid" themselves : www.arrid.com.au
Regards
Neil

Ray,

I have one of them and itis great. In my view while not cheap they are one of the best things to include in any dual battery setups. I am amazed by the stories on the forum about people and their dual battery setups, quite a lot is utter rubbish and not supported by electrical theory.
I have had many types of dual battery systems for over 30 of 4WDing from simple relays or solenoids to the BTC20 with the exception of Redarc products so my comments do not include them.
The problem with dual battery systems boiul down to 2 fundamental issues; (a) to get the right voltage at the aux battery (b) to accomodate the different charge characteristics of dissimilar batteries.
I found the simplest solution and by far the best was to have a 2nd alternator, a pretty easy project on varios V8 Landy vehicles I've owned. Not all that easy on other vehiles though and the rason for buying the BTC 20.
The BTC 20 is a small switch mode powersupply device which provides a fully independant voltage and current supply, it can provide the correct charging voltage to the battery it is connectd to despite any voltage drop between it and the source of power. It allows the vehicle alternator to charge the standard battery without any loading or masking by the aux battery. The use of this product means vehicle alternator output is constantly available the the BTC20 to charge the aux battery unlike most other systems that wait until the main battery is charged before placing the aux battery in parallel. This could be important on short runs where the time spent charging the aux battery could be quite reduced.
Any system which simply connects 2 batteries together either after charging one first or from start up in my view is totally flawed. A system [such as some Redarc models I think] which charges one battery and then connects the alternator only to the aux battery is better although this can be a problem if the aux battery has considerable load and drive time is short.
Some people get away with inferior systems and then proclaim the wonders of this or that product but eventually the aux battery will give up the ghost and you see posts questioning the performance of this or that make and type of battery. I'd say in 99% of these cases it is because it wasn't ever getting properly charged.
The maths are pretty simple, take 80 amphours out of the battery and you need to put 80 back in, with a charge current of 10 amps that is 8 hours continuous driving !
Some people think because the vehicle has an 85 amp alternator there is this much current available to charge the battery. This is absolutely not the case and in fact the charge current rapidly drops to a tenth of it max output.
With a BCT20 the current can be maintained at close to the max charge current of the device of 20 amps. This ensures the aux battery will get plenty of current to recharge in a reasonable time.
I have no connection to ARRID or the 12v shop where it bought it but commend them for their service by phone from Vic.

Ken

Sounds very reasonable Ken however one importaint fact that has been left out of your calculations is the individual batterys' ability to accept a charge. My understanding is that deep cycle batteries will not accept the rate of charge that a cranking battery will. Another variable that effects rate of charge is the residual charge in the battery when charging is commenced, the lower the voltage in the battery at the time charging commences the slower the battery will accept the charge, to the point where if you use a lead acid deep cycle battery and you flatten it to say 10.4 volts the battery may take as long as 100+ hrs of driving to recharge the battery, as it only offers thew battery a high ampage delivery not the trickle charge it needs to start the charging process (ever heard of 3 stage battery charging! There is a reason).
I believe the twin charge you talk of is a 12v to 12v 3 stage charging system if this is the case it is the only unit that I have heard of that will charge a second battery correctly (14.4v. not the 13.8 that your altinator will charge it too). Good luck with your system I think that you will have many happy light years with your twin charge. I currently run a pirrana 180e system (a very expensive switch) which is reasonable and the only reason I didn't choose the twin charge was because I have to mount the system under the bonnet and there was a possibility of it getting wet and I don't think the twin charge would like that.

Hi Drivesafe, amused you may be but hardly more amused than I was to read that you believe dual battery systems have worked for decades. You have obviously paid little attention to the many posts here about dual battery systems, problems with deep cycle batteries failing after short service and the many other problems encountered.
I was not saying any particulur device was now needed although my first post supported one of a few products that actually can work. The main thrust of my posts were to provide some factual info on why there are problems. If you are so intent on looking for something to rubbish about the attempt to provide some useful info then feel free. Many members look to the Forum to solve their problems. Many, including some of the 'experts' offering advice, have little knowledge of electrical theory and as a result provide doubtful advice. Endorsements of solutions solely on the basis the writer has not experienced a problem so far are not helpful. With an understanding of the requirements to correctly and fully charge a battery people will understand why there can be problems and can make up their own minds on the product they use and the amount of money they spend and compromise if required on the cost versus usage pattern.
In my case the Twin Charge was about the same price as the ARB product. I learnt the hard way, paying once for something which does work and once for something which doesn't. I was lead to believe by some turkey the ARB item charged the batteries independantly which like many similar systems it clearly doesn't.
If you think you have all the answers give your views to the members in a helpful way however if you think dual battery systems have been working for ages I doubt you have much to contribute.

Ken

Drivesafe you obviously are a very imaginative reader. Nowhere have I said you [or any one else for that matter] have set out to deliberately decieve anyone. What I did say and absolutely stand by the comment, is that much of the advice on daul batteries here, your limited contributions included, does not explain the limitations of systems which merely connect 2 batteries together. It is not just a question of wire gauge and having things in 'good nick' as I'm sure you would realise if you thought for a moment and used your 30 years of whatever it is you have done. Are you seriously saying there would not be sufficient voltage drop on a cable and connectors to a battery in a camper trailer or some distance from the current source to prevent an auxilliary battery charging fully ? Do you understand that even 0.2v drop in charging voltage will mean the battery cannot reach full charge. What I also said was that some people, possibly you included, that have long charge times versus short discharge times never experience a problem. It is this rather than any product that I'm saying is the probelm. Just because these users don't experience problems with a less than fully charged battery doesn't make the system suitable for all applications and that is where you and others are misleading people.
As for having one product fail that is also your imagination at work.The ARB device did not fail, it worked perfectly, as designed, connecting the aux battery to the main battery once the main battery reached a preset voltage. I'm sure it is well made, it had all the necessary bits and the wire gauge was more than adequate. The probelm is as I have said consistently in all my posts and what you seem to have missed is, it only connects the 2 batteries together. Ask yourself, which battery will have the higher terminal voltage and what will the alternator do when it sees that voltage.
Unless there is no voltage drop at all in the system connecting the 2 batteries, all of it, not just the cable but all connectors, switches/solenoids, the aux battery will always be at some voltage slightly lower than the main battery. Not a lot but enough to ensure it never fully charges. Simple, physical, fact.
Now tell me again how it doesn't matter where the aux batteries are and we can all have a good laugh. Amusing, yes I'm amused.

PS. Maybe if someone invented a device which overcame the voltage drop of the connecting system, remotely sensed the voltage of the aux battery and adjusted its output voltage to ensure the correct charging voltage was applied and could pump out a decent charge current the problem would be solved. I think a good name for it might be something like Twin Charge.

Ken

AHHH dear, porof again of the dangers of a little knowledge ! For the absolute last time consider the circuit conditions.
The issue isn't the alternator output voltage which is relatively constant. It is the difference in voltage at the 2 batteries. With the remote battery always being subject to some voltage drop along its connection wire the main battery will have control of the alternator output. Your example of a 2v drop is a bit extreme for even a half decent wiring system but lets say this is the circuit conditions that apply.
After a short time the main battery which has only had to start the motor charges and the alternator current will reduce to quite a low value, just a few amps because surprise, surprise, it sees a battery voltage of 13.8v or greater. It thinks it has done its job and charged the battery and to avoid boiling it dry it reduces its output current.
Meanwhile back at the aux battery, it hasn't had anywhere near enough time to replace the charge it has been providing to run its load. Remember it may need to replace 50 A/hours or more, but what happens is the current is cut to just a couple of amps. Lets say the aux battery gets all of this current, unlikely but assume it gets it all; and assume during the time it took to charge the main battery the aux got say 10 amp/hours replaced, leaving a further 40 A/hour required to become fulled charged. This means the meagre 2 amps the alternator is now putting out is going to take 20 hours to replace the remaining 40 A/hours required.
Now do you see why I say the aux battery wll never get charged fully.

Regarding 'pumping 20 amps into a battery' there is nothing in what I said about pumping anything anywhere. What I said, and again your argumentative nature has caused you to misinterpret what I actually said . What I said was "the current can be maintained at close to the max charge current of the device of 20 amps". It means that if the battery can accept this relatively high level of charge it is available. Many deep cycle batteries can easily take this level of charge current but never receive it because of the limitations of a charging system which only ever connects them to a near or fully charged battery.
I do happen to know exactly how a battery charges and despite your silly misunderstanding of what I actually said, you are correct in saying the battery is the prime determinant of how much current is consumed, however I hope you accept it is only one of the important factors in the charging cycle. If the current available from whatever charging source is used is less than the value that could be accepted, or if the charge current is not applied for sufficient time, clearly a battery will not fully charge. In the circumstances relevant to charging auxilliary batteries in recreational applications current and time are far more important factors than the maximum current the battery can accept.

Your last 2 paragraphs are an amazing insight into your lack of appreciation of the problem. Again you are correct in saying the alternator only tries to achieve a pre determined voltage level and produces current in an attempt to achieve this voltage, and yes the alternator doesn't care where the current goes. This is all irrelevant hower and has nothing to do with the problem of charging the aux battery. At some point the alternator will achieve its desired voltage at the main battery and stop putting out the large current it needed to achieve the desired voltage. It does sense what state a battery is in and it is nonsense to suggest otherwise. If what you say is correct the alternator would never reduce its output current and it would boil the battery dry. Of course the alternator can sense what state a battery is in. It is a fairly crude sensing I admit, just on battery voltage but it HAS to know when to stop charging and does so on sensing the battery voltage. This is not the best way to determine if a battery is fully charged but these are relatively simple systems and for replenshing batteries with short duty cycles such as cranking batteries it works OK. Unfortunately in dual battery systems it is always the main battery that determines when the charge current is reduced.
You need to look beyond your knowledge of simple automotive electrical systems and realise the different conditions that apply in multi battery applications. Don't shoot the messenger trying to explain things you don't understand.
Hopefully our exchanges will have entertained the readers and demonstrated that a little knowledge, selectively employed is dangerous and misleading. With mainly acknowledgements of how my comments have helped explain things I feel my job is done. Probably haven't convinced you but your selective asessment of information and ability to misinterpret things makes it hard.

Ken

Scene Limit #1: If the resistance of the cables between the 2 batteries = 0 ohms,the Arrid will perform "perfectly" in charging the Aux because the alt sees the nett load[ of the 2 batts] as still requiring charge, hence providing 14.4v + current.

Scene Limit #2: If the resistance of the cables between the batteries >~ 0.7ohm the Arrid is no longer "functional" [assuming it is trying to supply 20A to a discharged battery]

Conclusion:
[1]If the cabling is heavy enough, the Aux will charge. With the Arrid the charge time is shorter.
[2]If the cabling is "poor"[<~0.7ohm],the Arrid will perform....just not as effectively.

Nothing beats having heavy cable.

- "the main battery will have control of the alternator output."

- "After a short time the main battery which has only had to start the motor charges and the alternator current will reduce to quite a low value, just a few amps because surprise, surprise, it sees a battery voltage of 13.8v or greater. It thinks it has done its job and charged the battery and to avoid boiling it dry it reduces its output current. "

I have never seen an Alternator Regulator that senses the state of Battery charge and compensates the voltage. An Alternator puts out a CONSTANT voltage regardless of whether battery+lights+anything is drawing 1 amp or 50 amps.

The reason that the voltage drops after some time on modern Alternators is the Temperature compensating circuit. As the sensor INSIDE THE ALTERNATOR heats up due to hot air coming off the radiator and heating from the losses in the Alternator, the voltage drops e.g. 14.2 cold to 14.0 hot. This is done to avoid overcharge at high battery temperatures and underchare at low temperatures.

Mike
Licenced Electrical Contractor
35 years experience as Professional Engineer
35 years experience designing, building, testing and using a large variety of electronic and computerised equipment.