Changing Battery, OBD2 Battery Keeper 1998 v70

[MrAl]

Hello there,

Has anyone done this with this or similar Volvo?
This is where you use a 12 volt power source (like another battery) and a little cable with alligator clips on one end and an OBD2 connector on the other end, and you plug it into the OBD2 connector on the car and use some sort of 12v battery or other power source to power the computer while you completely remove the normal car battery and change it out for a brand new one. This device supposedly keeps the computer memory intact so you don't loose the computer OBD2 readiness.

What i would like to know exactly is what did you use for a power source to keep the computer memory alive?

I ask about this because there could be an issue that comes up if the power source has a battery voltage that is higher than the car battery before you replace it. If the power source is another 12v battery it could be 12.8 volts while the car battery is 12.0 volts or even less because it is not that good anymore, and with a voltage difference of even just 0.8 volts there could be a significant current through the OBD2 connector and wires. It looks like a sure fire way to get this done though if it can be done right, so i might attempt it at some time in the future once everything is working normally.

Any ideas?
Thanks.

[abscate]

Just use another car battery and connect it, then disconnect the Volvo battery. A few seconds of current from battery mismatch won’t hurt anything

The non-continuous OXS and cat sensor are the ones that take the time to go ready as they need the over run condition with no fuel injection to test the response of the sensors. The evap test won’t run until those sensors clear. Covered in other threads. Work on a solution to kerp the battery charge up so you aren’t always dropping readiness and life is a lot easier

[MrAl]

Just use another car battery and connect it, then disconnect the Volvo battery. A few seconds of current from battery mismatch won’t hurt anything

The non-continuous OXS and cat sensor are the ones that take the time to go ready as they need the over run condition with no fuel injection to test the response of the sensors. The evap test won’t run until those sensors clear. Covered in other threads. Work on a solution to kerp the battery charge up so you aren’t always dropping readiness and life is a lot easier

Hello again,

Oh i can put up with the Evap not flipping to ready that's been the case ever since i had this car, but the other ones usually flip by now that's what is strange this time. Well, there are those three that flip fast, they did flip, like the gas/air mixture monitor or something like that. There are six that wont flip, one is the Oxy sensor the other is similar. Secondary Air did not flip yet either and that usually flips after some time, but another strange thing is i actually HEAR that test being run as i can hear the secondary air pump running during almost every run of the car. it must be testing that part.

[jmartin919]

What the heck are you two talking about and do I need to understand it for my S70?

[scot850]

When you disconnect and then reconnect your car battery on these models, it wipes your settings in your engine and you have to run various cycles of driving conditions for the car to relearn them. Especially if you touch the +ve and -ve battery cables (when removed from the battery!!).

What Al is asking is if you disconnect your battery you lose the settings the car has learned and are needed for their inspections.

I was thinking, can you back feed through the cigarette lighter/12V outlet using a jump pack to save the values when disconnecting the main battery?

Neil.

[jmartin919]

Or just hook jumper cables to another battery.

[MrAl]

Or just hook jumper cables to another battery.

Hi,

It's hard to do that when you go right to the battery connectors because you then have to remove the connectors so you can replace the battery and if even ONE of those jumpers pops off of the connector, you lose all your OBD2 monitor settings. They all go from 'ready' to 'no ready' and you can't get through inspection.
That is why you might go through the OBD2 connector, so that he connection is more permanent as you are changing the battery.

Going through the OBD2 connector however means you will be pumping current through the OBD2 wiring, and the wiring can be very light gauge as low as #26 AWG. Using #25 as a reference, that size can handle about 0.5 amps comfortably. The resistance is about 32 Ohms per 1000 feet, which means 1 foot would have a resistance of 0.032 Ohms. Assuming the OBD2 connector has 5 feet for positive and 5 feet for the negative wires, that's a total of 10 feet which comes out to 0.32 Ohms total.
Now if we push about 1 amp through that wire it can probably handle it for a short time, maybe even a bit longer.

Ok, so now we want to connect a battery to the OBD2 connector while the car battery is let's say 11.5 volts and the OBD2 battery is 12.5 volts. That means we have a 1 volt difference. Now 1 volt across 0.32 Ohms is about 3 amps, which is pretty high for #25 wire. The question then becomes how much does the wire heat up and how much do the connectors on all ends heat up. If they heat up too much it could ruin the OBD2 wiring or the connectors.

The saving grace is that the old battery will probably have a somewhat higher internal resistance than a brand new battery, so the voltage of that battery may rise quickly to a higher value. Let's say it goes from 11.5 volts to 12.0 volts almost immediately. That means now we only have 0.5 volt difference, which means now we only have about 1.5 amps going through the OBD2 wiring. That may be ok, but that's only if the voltage does rise that much soon after the connection is established.

One of my reasons for bringing this up was because i happen to have a nice LiFePO4 battery that has terminal voltage of about 13.5 volts. It would be very convenient to use that because it's about the size of a 12v 7 ampere hour lead acid gel battery. It weighs about one-third as much as the lead acid counterpart too, so it's easy to carry up and down stairs.
Unfortunately, that would mean it could be a 2 volt difference, which would double the current estimates above.

Maybe the answer is to add a little more series resistance during the initial connection and see what happens to the battery voltage.
Since the resistance is so low being around 0.32 Ohms, if we add a 1 Ohm resistor that would lower the current to less than 2 amps, and a 2 Ohm resistor would lower it to less than 1 amp even with a slightly larger voltage difference. That could be the trick. We can then measure the car battery voltage and see that it has gone up a little, and once we disconnect the car battery the voltage should go up to nearly the full smaller battery voltage, in this case 13.5 volts, as the electronics probably only draws a small current like 10ma or something. 10ma through a 2 Ohm resistance only drops about 20mv which is almost nothing compared to a car battery voltage.

I guess this would have to be tried but it does seem like it should work well.

[jmartin919]

How about discharging the new battery through an old headlight until it drops to the old batterie's voltage?

[xanthefin]

Well i am not gonna bring much value to this topic but just wanted share i been using this kind of charger in my Volvo to keep battery in level when if doing much only city cruising specially in winter for years. About the wires there is fuse too which can be fried but i haven't got that done yet.


If i gonna charge dead batteries with dum dummy charger without smartness i might end up using for first minutes a car headlight bulb in middle of lead as resistor.


For then if someone designing actually OBDII safety up keeping device here i would warmly take a note arduinos with zener diodes which switches voltage source automatically if goes above voltage x.

[BEJinFbk]

Couple of things:

I’d bet that the electronics on the car aren’t
anywhere near as picky about voltage as what
I’m seeing in this post. You can be fairly certain
that Volvo has “Owner-Proofed” things pretty well.
The first thing that any decent engineer does is
build solid voltage conditioning into the front end
of any processor that’s going to spend it’s life
in a vehicle.

The cig lighter is great for plugging in a back-up
12 volt supply in a lot of older cars, but the Volvos
( and MANY other vehicles )are connected to the
ignition switch, not a constant battery source.

As mentioned, OBDll appears to have B+ on pin 16.
Triple check that. You probably don’t want to point
12 volts up the wrong pin.
Ground appears to show on pins 4&5.
.

88973F01-DFD2-4B66-BF0F-C7F9B3E21FB6.jpeg (136.77 KiB) Viewed 522 times

And how many people am I gonna tick off
if I bring up the accessory connector again?
It’s tailor made for this assignment!