Hi again RB, checked battery today; 12.48V after standing 2 days, so 3/4 fully charged. After start up charging settled at 14.04V.
Switched on headlights and it kept to this voltage, balanced load even at 1000rpm, but didn’t exceed it when revved which was impressive.
Didn’t run it very long and I’d expect it to drop a bit when the battery charged up further but will check on a run.
Should say I was metering directly across the battery terminals and the meter checks good against 2 others.
Hi Rich M
I havn’t been ignoring you just that I wanted to do a proper test.
I’ve just driven 225 miles to Blackpool up the M6,M40,M42,etc. The battery was fully charged to start with, however even at the end the charge rate has not dropped below 14.6 volts. So not very good then! is this a fault or just the way it is? Either way with the regulator in the ECU there is nothing I can do about it.
R-b
14.6 volts is about what my Mk2 runs at.
Your MX5 was fitted from new with an AGM battery. The charging rate is therefore higher than for a ‘run of the mill’ lead acid battery. Nothing at all abnomal regarding the charging rate on your car. Up to 15v. is fine, according to the Mazda manual.
Regards Geoff Peace.
Hi RB, Geoff, all,
As I understand it an AGM battery is lead/acid and although it has some advantages it
should be charged at the same voltages as a standard battery. The manual
does give 13-15V as the charge rate but the 15V would be short term and
should fall back ime. The alternator current test in the manual is done with a
fully charged battery and shows 13.5V. That said, if it’s charging at 14.6V continuously without problems then you have nothing to worry about I suppose.
Forgive me if you’ve already done this but, first have you checked your meter against a couple of others as we are talking tenths of V and some meters are pretty dire accuracy. Secondly, where and when did you check voltage?
Even with a full battery if it’s checked just after start up the charge rate will be 14+V. It needs to be checked during or after a long run without turning off and re-starting.
Arranging test leads from the battery terminals to the meter inside the car with you on a run would be the ideal way. (You may be able to do this easier using a cigar lighter plug but you’d need to check first that the local circuit load wasn’t affecting the voltage on the cigar circuit.)
Rich
I have checked my digital meter against calibrated units in a test lab and it is within 0.1 volts of their kit so no problems there.
I have checked a number of Mk1’s and Mk2 and some other Japanese cars with small batteries and they are all in the 14.5 to 15 volt region when charging, the MX5 especially not going donw to a normal say !3V after a run.
OTOH my Impreza with a normal sized battery is nearer the 13.5 volts charging just like we are normally used to. I will check my Mk3 later and post the results but as it uses a normal battery, I guess nearer 13.5 volts.
I would not check the voltage in the car as there will be some voltage drop so not the best place but as rough references possibly.
Hi Eddie,
That’s interesting and it will be good to see how your Mk3 checks. There should be no problem checking inside the car with longer leads btw as modern digital meters are very high impedance typically 10M so draw almost no current (= no voltage drop on the leads). The ciggy circuit could be a different matter of course and may be part of a locally loaded circuit.
It would be good if you can check your Mk2 with leads from the boot to a meter in the car and monitor it as you drive for say an hour and see what actually happens.
You have more exp of these cars than me although I’ve followed various charging and battery stories for 40 years since the dynamo days. There was a period (around 10 - 20 years ago?) when charge V was tweaked by some makers in response to new battery technology and at least one maker found theirs too low for some of the newer batts, to do with low maintenance/calcium types iirc.
Anyway, always good to hear what you find.
Hello Rich.
Firstly, yes all my meters are calibrated so they are accurate, but your question is a valid one. Secondly an AGM battery should be charged at a higher rate than an ordinary ‘wet’ battery. Anyone with a Ctec charger will note that it has a setting for ‘ordinary’ charging and a higher one for low temperatures and AGM batteries. The charging rate for a ‘wet’ battery should be 13.6 - 14.4 volts. An AGM type ideally 14.8v but, as the Mazda manual states up to 15v is fine. The point where this is checked, I agree is important. Very few people conduct a voltage drop test on both the +ve and -ve lines. If the line drop is more than 0.2V, some would say up to 0.5v is acceptable, I prefer 0.2v, further investigation is necessary. So the difference in voltage at the alternator terminal and the battery terminal will vary. I test between the bolthead for the 100A main fuse, in the fusebox under the bonnet and the alternator frame, my reading is 14.8v so nothing to worry about, this will fall to around 14v. as I switch various items to load the alternator. I do not know how far back you go Rich but modern alternators are far more precisely regulated than dynamos with their external mechanical regulators. The cut outs were mechanical and were only set to cut in at 13v. if they cut in below that, at tickover speed say, the battery would try to drive the dynamo. They also were regulated to16.6v for the RB106 and 108 then along came the RB 340 at a mere 15.5v These high charging rates reflected battery technology at the time and would regularly blow bulbs and lead to shorter battery life. Drivers today do not generally realise that in the fifties if a battery lasted two years you were jolly lucky. I have to confess, when I first bought my MX5 I checked the charging rate and thought it was too high, a little research showed that it was not. I think, and I stand to be corrected on this, that the Mk.3 has a conventional battery so I would expect the charging rate to be lower, but I do not have any figures.
Regards Geoff Peace.
Mk3 after starting car 14.3volts at battery terminals. Half hour drive but still running no switch off 14.1 volts.
07 Suzuki Jimny Jeep 15.3 volts after starting and 15.3 volts after 30 min run, with AC, headlights and HRW it was still 15.1volts. Battery original and no problems.
My 1600 was messing about charging/not charging.Had a look,the bottom bolt that goes in from the front has fell out and is laying on the undertray,thats prob why they put them in from the back ( and it makes it awkward for diyers) as a bonus.
eamonn
Hi Geoff, Eddie, all,
You go further back than me Geoff but I had dynamos, early alternator with separate regulator, and the later combined alternators. RB340 is the only reg I remember, on the MGBs, and it worked well but a Matchless with dynamo was hopeless and often boiled the battery ruining several pairs of jeans!
The MX5 Mk2 manual,
and Mk3 manual
both say 13V - 15V checked at the alternator B terminal. That may be a little lower at the battery terrminals due to resistances as you say Geoff.
Both these cars use alternators wired backed to the ecu and I imagine that would give “mapped” charging depending on load temp etc otherwise I can’t see much point doing it. All the voltages we found are in the range given as OK so perhaps we should just accept all is well.
Perhaps RB could check the temp of the batt after a long run to be sure it’s not overheating and if not then just trust all is well.
Hi All
I’m back after another long drive and no the battery does not get hot.
What is happening is that the charge current drops to a low value. How low I don’t know yet. I have set up a variable DC supply, connected to the battery and I am monitoring the current whilst manually keeping the volts to about 14.6.
I have looked a number of articles on the internet with the conclusion that what hurts AGM cells is overcharging when fully charged. Fully charged being defined as the terminal volts rising to 14.6V at the end of the absorbtion phase. Overcharging being defined as a current greater than 0.01C. “C” being the the battery rated capacity at the 20 hour rate which in the case of the Panasonic battery is 32Ah. Therefore 0.01x32 = 0.320Amps At currents above this value (and only when fully charged) Oxygen starts to be evolved from the positive plates. If the internal pressure exceeds the valve setting (about 2psi) this gas is lost and the capacity of the battery declines. So regular overcharging is to be avoided.
Back to the test, after nearly 4 hours charging the charge rate has dropped to 215mA at 14.6 V and only 250mA when the voltage is raised to 15V. All this is less than 320mA so theoretically no problem.
R-b
Hi RB,
That’s good news on the batt temp and it seems there’s no problem then as you all get similar voltage readings.
Your AGM batt info seems to back that up too and regular vented batts have a gassing voltage of 14.7V so may be similar.
Looking for a commercial view of this, my smart charger can be switched to 14.4V for sealed of any sort and 14.7V for open vented types. It has an accurate digital voltmeter and in practice 14.7V is achieved within a minute or two of passing 14.4V when charging a vented type. It then switches to 13.2V for either type which can be maintained indefinitely as float. I put an ammeter in circuit and found it continued at 700mA on the 13.2V setting and gradually fell to around 200mA. It may have gone less but I didn’t check further.
Mk2, Mk3 and Mazda 6 use the ecu to control the charging and their manuals all say 13V - 15V so I suppose it will vary under some conditions.
I looked charging up in the training section of the Mazda 6 manual but all it has is a diagram confirming rectification in the alternator as usual then connection to the ecu instead of the regulator for what looks like similar physical control via the field current. It doesn’t give any idea what data is used to set the charging rate. The only text says, “The voltage regulator has been eliminated, and generator control is carried out by the PCM.”
There’s lots of stuff on the web but this one is worth a read if you’ve not come across it. Even the manufacturers don’t always agree and the reasons here show it may be plate chemistry as well as sealing and construction.
Para 5 and 6 for example explains a 14.8V charge rate set by GM for lead calcium types which hardly gas. Briefly, adding calcium decreases gassing but needs a higher charge rate and amongst this technology are hybrid constructions and a balancing act to perform re charging.
It was written a while back and AGM batts aren’t mentioned by name but the 5th para from last talks of a plate type which is happy at 15+V using silver (which Bosch are also using in their high end batts today).
So there are different recipes as well as constructions. For obvious reasons aftermarket suppliers will be careful to make their product suitable for as many cars as possible. It seems likley that most batts will be ok on most cars (I checked the Varta list and it says nothing special for our Mk2 and Mk3 cars, a B32 on ours has been great for nearly a year and also changed our Mazda 6 batt for a standard Banner 2 years ago which works very well) but there may be the odd less than perfect match out there for reasons shown in this page.
Hi Rich M, IHT and all
Yes I have read the article and it does adds a bit of useful information but as you say a bit out of date. I am finding that an all to common a problem with the internet. An awfull lot of infomation relates to “open vented LA” and/or deep cycle AGM cells for the American recreational market. Not enough about “no maintenance” Ca/Ca cells or AGM’s for car use. When was the last time we saw open vented cells in cars, 20 years ago, 30? I am going to try and put together a table of voltages of all the different types.
Back to car charging problem. The function or a car charger is to get as much charge into the battery as quickly as possible and are only expected to charge for short periods which the battery can tolerate. This is quite different to the use of mains chargers to regulary recharge deep cycle batteries or keep car batteries topped up which require controlled charging and accurate floating.
But I do have two questions:
What controls the max current into a discharged battery, the regulator maintains a constant drive voltage and does not know where the current is going. It could be screen heaters or headlights or all into the battery. The alternator could easily produce say 40-60A, I don’t know what the exact rating is. Surely this is going to damage a flat battery if kept on for too long.
The OP IHC is only getting 13.6-13.8 Volts from his alternator, surely this is nowhere near enough to fully charge a any battery. A No Maintenance battery requires about 14.8 V to absorb full charge, so depending on car usage in the winter this might be a problem… There is posting on the miata site that says that in the NC cars the charge voltage is temperature dependent and controlled by the air intake temperature. So IHC there is something you can do and that is to do a scatter graph of voltage against intake temp and see if there is any correllation. It may be that the temp was high because you were stationary and that is why the volts are low.
R-b
Hi RB, all,
It shows a 20 year period of an on-going saga up to the MX5 Mk1 era. The battery people make a batt that’s better in some ways but if it doesn’t conform to within 0.5V of what’s already out there it may cause problems. I was working in the parts trade when we got those first low maintenance types over here in winter 1975 and had to uplift loads from garages. The better garages who tested batts and checked charging rates for customers were soon telling us that they wouldn’t charge. I don’t think it’s that they won’t fully charge at the slightly lower voltage, more that the lower voltage causes slower charging which doesn’t keep up with loads on the car so the battery becomes depleted. It might not happen today with higher current alternators.
Re. the wet open vented type, many (all?) new Mazda’s have them. We have 2005 MX5, 2006 Mazda 6, and 2011 Mazda 2, and they all came with them. I replaced the Mazda 6 with another wet open vented and the MX5 with wet “sealed”, so they’re still very much with us. A table would be a good but I think you’d struggle as the exact specs aren’t out there.
Yes the web info about batts designed for recreation and alternative energy sources can confuse things.
As I understand it and probably wrong(!)… the battery draws whatever’s available and bulk charges up to the voltage allowed by the regulator. It’s laggy and can get a surface charge to 14V or similar before being fully charged due to acid non-circulation etc. As the battery voltage increases the potential difference (V) between it and the source decreases, so the current (I) into the battery’s resistance ® also decreases, I = V/R, and stops when they equalize. There will be additional resistances making up the total load impedance from cables and connectors etc which will also affect this.
The Mk1 regulator quotes 14.1V - 14.7V. The Mk2 and Mk3 say 13V - 15V. That makes me think the reg may adjust voltage to control current under some conditions. A constant voltage reg would not do that but if the voltage is adjusted up or down the current will follow and that may be the point the simple reg becomes a smart one by reacting to external data.
Here’s a good page on car voltage regulation about halfway down.
IHC was taking voltage from the OBD 12V pin I think, the circuit may be under local load at that point, see our posts earlier in the thread. If so his results may be a relative comparison between faulty and fixed, rather than a check of what the batt’s actually charging at.
Rich
Hi Rich
Thanks for that link I hadn’t come across that one. Good basic stuff very interesting.
It’s the no maintenance Calcium alloy batts that require the highest voltage which is why you were getting customers complaining about flat batteries no doubt mostly in the wintertime.
Any table would be an average of all advice, as you say there are no definitive answers but it might help somebody.
You are quite right and that was what was worrying me. With a discharged battery having say 11V volts, the alternator being regulated to say 14V, the driving voltage is 3V. The only limit to the current is the circuit resistance which at quick calculation/guess ( more research requ) is about 20 mOhms for the cabling and 20mOhms for the battery. This gives a current of 3/0.040 = 75A. This is a worringly large figure and I hope it’s not true.
Apart from starter, headlights and screen heaters most currents are small (<5A) and the volt drops on circuits are quite negligable so measuring battery voltage on any circuit at the front of the car even with a boot battery will be very accurate. I also use an OBD reader to monitor the volts and they are very accurate too. The bluetooth signal is strong enough to connect to my phone when I’m sitting in my armchair. I can monitor the volts as I type, how about that for technical progress!
Cheers
R-b
Hi RB,
Yes there’s very little specific info out there. I searched on the good old 16ACR, Lucas’ first(?) integrated alt and reg from about 1970, 34 amps, went on almost everything Brit at the time.
Batts are fine when we jump start so that shows they can take it, a potential 200A there I suppose. I think your example is fine. The batt will try and draw 75A and will drag down the supply to a level it can manage while it charges and its voltage comes up to reg voltage. The reg may have current limiting or temp control to protect the diodes and windings.
I agree about small drops from light loads but it’s difficult to know so I’d check any remote measurement point against the voltage at the batt terminals. We only need 0.1 ohms in the circuit to drop 1V if the load is 10A and that’s not hard to imagine with petrol pumps, fans, lights etc.
Here’s a good link if you have time 
Forgive me for saying so, but I think your logic is somewhat academic rather than practical and as a result you appear to be going off at a tangent, several it would seem. The charging current of a battery is governed by its state of charge and the charging voltage. A flat, and I do mean flat, battery will try to draw excessive current. Sometimes it is necessary to start the charge at 6v. until the battery establishes some internal resistance and is able to take a 12v charge. Although usually if a battery is so far gone it is scrap. A charging voltage of, say 14 .5v will supply the battery with the correct charge rate. Bear in mind that it is being charged at constant voltage, not constant current. To charge at constant current the voltage must be increased as charging proceeds. With constant voltage the current decreases as the battery becomes fully charged. The new generation of ‘smart’ alternators were pioneered by Ford around ten years ago to be used with silver-calcium batteries, and no other type of lead acid battery. The main difference is that the alternator output is controlled by the ECU rather than an in-built regulator. Now, this is very different from the oft quoted ‘the regulator is in the ECU’ It is not! the output regulation has progressed beyond the conventional regulator and is now extremely sophisticated. On starting the ECU checks the engine coolant and air intake temperature, and, if cold, will boost the charging rate, reducing it as the temperature rises. It is not used to compensate for high demand despite what many people think! Compensation for demand is dealt with by the ECU sensing a charging voltage drop and will endeavour maintain charging voltage. In the end all this is purely academic. If the alternator is faulty the only course of action is to replace it. Detailed analysis of the cause of the exact cause of failure will not alter the fact that a replacement is required. As an afterthought Mazda say that the Panasonic battery can be charged at 20A for half an hour provided it is partially immersed in a water bath to prevent overheating. Charge a battery at 75A and stand well back I do assure you. Personally every time I hear of car batteries being fast charged I am inclined to buy more shares in batttery companies!
Regards Geoff Peace.
Geoff
Well I’m not sure what brought that on, however if you think I have got something wrong or you have some practical real measurements then please let us all know.
I don’t disagree with most of what you wrote but I’m not sure what point you were making.
Incidently I did not advocate high high charging currents, please read what I said.
R-b