Prado 120 Diesel Alternator Voltage

Submitted: Monday, May 17, 2010 at 22:28
ThreadID: 78554 Views:13754 Replies:6 FollowUps:7
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I have the 2007 Common Rail Prado. Maximum voltage it is set to generate is 13.6v. Not enough to properly charge a 110a AGM or other sealed deep cycle battery. Apparently, they need at least 14.2v. I have read some threads that stipulate 14.7v.
Because of my alternator's "smarts", I have had to compromise and settle for a 75a wet cell under the bonnet. It can receive charge from my alternator, but significantly reduces my ability to stay at the one camp for more than 1.5 days - ie without going for a long drive.
I have recently come across the Redarc BCDC1220. It is designed to boost power to batteries in a trailer (ie - to compensate for power dropping while travelling from alternator to trailer batteries).
Apparently, it can also be used in my situation, to provide the power adequate to properly charge a nice big AGM.
I'd be interested in comments on the Redarc BCDC1220. Is it the solution for me. Will it fair dinkum deliver charge to an AGM. Will it survive under the bonnet - or must it live inside the vehicle. Will matters be complicated or enhanced when I buy my camper caravan - kitted out with its own solar and said to have sufficient capacity to not only charge its own 2 x 120a batteries, but also send charge "up stream" to the deep cycle under the bonnet.
Please use words with one syllable (or less)!! This stuff makes my head hurt!
But I'd appreciate your considered views.
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Reply By: Member - Stuart P (WA) - Monday, May 17, 2010 at 23:02

Monday, May 17, 2010 at 23:02
your alternator is set at 13.8v - 14.5v and when under bonnet temps rise will fall back to +- 13v
some new fords go back to 12.7v when temps rise to an overheat level
AnswerID: 417085

Reply By: Member - desray (WA) - Monday, May 17, 2010 at 23:17

Monday, May 17, 2010 at 23:17
Item number: 320531837221 E-bay boosts the alternator output for Prado
AnswerID: 417087

Reply By: ChipPunk - Tuesday, May 18, 2010 at 02:26

Tuesday, May 18, 2010 at 02:26
A general rule is that all 12V lead-acid batteries should be charged at a maximum of 14.4V (long-term) at normal temperatures. (With voltage dropping as temp increases - I think 0.1V per 10 degrees C, but I'd have to check....)
Once full, they are supposed to drop to 13.8V.

Someone once wrote that diesels charge at a lower voltage, but they never explained why...
But since a vehicle's electric system (voltage) is based on its battery and not its systems (if 12V or 24V etc), whether diesel or petrol shouldn't matter.

If you have a classic alternator (not the newer DP or ECU-controlled/interactive types), then....
Many trick their alternators into higher voltages by inserting diodes in their sensing line. Easily done if the alternator as an S = Sense wire (like 2-wire Bosch SL alternators, or Nippon Denso SIL 3-wire alternators, etc) and usually as easy if it is a 2-wire IL type. (S=Sense = direct to battery +12V; I = Ignition +12V which is usually the sense input if no "S" is present; L = the chargeLamp circuit).

Single-wire alternators (with D+ output for the chargelamp) cannot easily be tricked.

A typical diode has a forward voltage drop of 0.6V, hence if that could be inserted in to you sensing line, the voltage should go from 13.6V to 14.2V.
A 20c diode like a IN4004 should suffice (1A, 400V) but silicon diode of 1A or more rated at 50V (preferably 200V) or more should suffice.

There are other ways of trimming the voltage, but diodes are simple and reliable. (And there are diodes with 0.2V - 0.7V forward voltage drops.)
AnswerID: 417095

Follow Up By: Allan B, Sunshine Coast, - Tuesday, May 18, 2010 at 09:24

Tuesday, May 18, 2010 at 09:24
Be aware that adding a diode to the alternator sensing thus raising the output by 0.6v may assist full charging of an auxiliary AGM or Gel battery but will also raise the charging voltage to the flooded wet cell cranking battery which may not be so good for it. Of course another diode could be added to the charge line to the cranking battery to drop the extra 0.6v but it would need to be rated to the maximum current and anyway is getting a bit messy.

The Redarc BCDC1220 (along with other dc-dc chargers) can be employed to provide 3-stage charging to AGM and Gel auxiliary batteries without disturbing the charge regime to the cranking battery.

Note however that the Redarc BCDC1220 has ambient temperature limitations. It is designed for use in a trailer to overcome line voltage drop. If installed in an engine bay it may be necessary to arrange heat shielding and ventilation.


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Follow Up By: ChipPunk - Tuesday, May 18, 2010 at 11:08

Tuesday, May 18, 2010 at 11:08
My interpretation was that the alternator was set to a max of 13.6V.

But as I wrote above, no battery (in general) should be charged above 14.4V. Hence the alternator should be set to provide 14.4V.
Whether this is to the main or other batteries is yet again dependent on desire & design. Many will set for the further battery (the biggest voltage drop) and then scrub off excess voltage for those batteries getting higher voltages (which is much simpler than boosting voltages!)

I set for the front battery and ensure I have acceptable voltage drops elsewhere - preferably with negligible earth drops (less than 0.1V, though that is my target for +12V anyhow.... as if!)

Which is worse - over charging or undercharging? They both kill batteries, but overcharging is generally worse through its gassing or direct damage to AGMs etc. And undercharging can be recoverable....
FollowupID: 687222

Reply By: Matt(WA) - Tuesday, May 18, 2010 at 08:31

Tuesday, May 18, 2010 at 08:31
Talk to Derek at ABR. He just sorted my old mans prado out for him. Get a plug in voltage booster of ebay as previously mentioned. But have a chat to Derek Bester first.


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Reply By: Member - Phil G (SA) - Tuesday, May 18, 2010 at 10:36

Tuesday, May 18, 2010 at 10:36
You'd want to be sure about what the charging voltage is before you embark on this path. Your alternator is temperature regulated - so the voltage drops when the alternator gets hot. Also, you may find your voltage drops a bit at idle.

Unless Toyota have changed the Prado alternators, I'd expect your voltage to be around 14.2 about 2 minutes after starting, and with the revs at about 2000rpm. 13.6V is what I'd expect in hot conditions.

Your AGM will charge at lower voltages - might take a bit longer. If you fit the Redarc, you will limit charging to 20amps. I use a clampmeter to measure current going into my batteries and after being used overnight, my AGM often takes 35-40amps at 2000rpm.

I wouldn't consider the diode option - too much at stake when you start delivering an extra 0.7 volts to all your vehicle's electronics.
AnswerID: 417128

Follow Up By: Allan B, Sunshine Coast, - Tuesday, May 18, 2010 at 10:46

Tuesday, May 18, 2010 at 10:46
Yes I agree with Phil.

Arrangements such as diodes to modify the alternator output are all "very clever" but can bring you undone at significant expense.


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FollowupID: 687220

Follow Up By: Wherehegon - Wednesday, May 19, 2010 at 08:07

Wednesday, May 19, 2010 at 08:07
High Phil, toyota changed the output on the prado D4D diesel's to 13.4, mine is the older shape (95 series) with the 1kz and is 14.4. Its a know problem with the 120 series with the D4D under the bonnet. Battery world sell a product which pumps the voltage up tp around 18 obviously controlled as to not cook anything. Regards Steve M
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Follow Up By: Member - Phil G (SA) - Wednesday, May 19, 2010 at 15:49

Wednesday, May 19, 2010 at 15:49
Gday Steve,
Thanks for that - I wasn't aware that the D4-Ds voltages had been dropped that far. I'll have to get a multimeter out on one to see what goes on.
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Follow Up By: Nomadic Navara - Wednesday, May 19, 2010 at 16:32

Wednesday, May 19, 2010 at 16:32
The alternator in modern Toyotas has been measured as low as 14.2 V when the engine is hot..

As stated above, all alternators have temperature compensation built into them. This is because batteries require different charging voltages at different temperatures. The voltages over 14.0 V that people are quoting are for cold motors with the bonnet up. However with things are closed up and you are down the road working hard, your battery heats up considerably. If your alternator voltage is not reduced your battery will boil dry.

My Navara alternator peaks at 14.4 to 14.7 V after starting. The voltage depends upon the ambient temperature. I have not been in freezing temperatures to see if it will reach 14.8 V. Within 20 minutes into a trip the voltage will be 13.9 V or less. I have noted it down at 13.6 V. One day when I went to retrieve my van, the voltage was 0.1 V lower on the return trip.

You can observe this temperature compensation yourself quite easily. Just get a spare cigarette lighter plug and make up a lead for your meter from this. Plug your meter into the cigarette lighter socket before you go on a trip (and turn it on.) Start your motor and observe what happens. Then drive off and monitor the meter every few minutes.

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Follow Up By: Wherehegon - Wednesday, May 19, 2010 at 19:09

Wednesday, May 19, 2010 at 19:09
Hi Peter agree with you inregards to temps etc but even a D4D prado is at 13.4 when its first started (cold) which means going buy what you say it will drop down alot lower when hot which is not real good when trying to charge a dual battery system. Not sure with the older ones like mine but even when its been running (normal temps) etc I have had the multi meter on it and still showing 14.4V ??? Maybe mine doesnt have the temperature compensation built into it been an older design (motor 1kz) ?? I would only be guessing to say wether it did or not. I have no troubles keeping both batterys charged up even though Im only doing roughly 18k a day. I put a duel controller in my mates d4d hilux only a few weeks back and his was reading 13.6 (motor normal running temp) but I believe the output on them is different again from the prado even though its a D4D ?? Regards Steve M
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Reply By: olcoolone - Tuesday, May 18, 2010 at 14:38

Tuesday, May 18, 2010 at 14:38
This will be interesting!

......can't wait.
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