# Thread: Doesn't hold standby power if I crank the car right away

1. I got the replacement M4-ATX in today and I swapped it out with the old one. I will replace the battery this evening and will run for a while to see how things are doing after some real world tests.

My assumption is that I still need some supplemental charging because of my driving habbits ie. taking lots of short trips through the day and long periods (2 or 3 days at a time) of just sitting.

I will cross that bridge though after I replace the obvious.

2. So I just got the new battery all wired up and I took some numbers.

@ the aux battery while idling it measures 13.51v (isolator connected)

The Aux battery measured 12.75v (isolated)
Standby current is roughly .3amps

We will see how far it is drained when I test it tomorrow

3. When I tested it the next day (roughly 10 hours) it was at 12.46v which is a marked improvement proving that the battery was indeed bad. After a short trip with the alternator charging it was back to 12.6

4. As I just wrote in the Why is my voltage dropping? thread - and elsewhere if not in this thread - a battery just off charge should be up to ~13.6V due to surface charge which then drops over 12-24 hours (with no load) to its nominal full voltage (12.67V, but say 12.6-12.7V).

13.5V is ok assuming that at idle, the alternator output has dropped. It certainly isn't discharging the battery, though long-term it is insufficient to charge it, and will wreck the battery without occasional equalisation charging (over 13.6V, usually ~14.4V) to blow away soft sulfation.

A modern alternator (ie, after ~1960?) should charge the battery with 13.8-14.4V with 14.2V being a common set point. That's at the battery - the alternator may put out more.
In theory, except for single-wire D+ alternators, the alternator should be sensing the battery voltage and hence the alternator compensates for voltage drops to the battery.
But the second/remote battery may see a voltage drop due to its distribution current & resistance (ie, V=IR where V is the voltage drop; I is the current along the distribution (fuses, cable, relay/isolator AND ground path) which has a total resistance of R Ohms.
As long as the remote batteries are generally near fully charged and occasionally see at least 13.8V they should be ok (assuming it isn't excessively discharged). (EG - normal batteries are discharged when cranking, but then have the trip to recharge. When the trip ends, its a fully charge battery. Audio-system batteries that are not fully charged are left in partial discharged states which cause sulfation.)

5. So it appears that i should have read all 27 pages of the "Everything you need to know about M4-ATX" thread

Apparently on page 22 it states issues with the IGN line being flakey in regards to holding standby current. The solution... Diode and a Cap on the IGN line.

So far switching from ACC to IGN has worked out well for me but i still have some unresolved issues:
http://www.mp3car.com/m4-atx/148142-...off-cycle.html

I will be adding this soon (RMA'd my motherboard)

6. One problem with the diagram as shown on that Everything You need to know about your M4 ATX link is that the Standby Battery (and LCD and M4-PSU) get about 0.6V less than the IGN +12V.

Not that that should be a major problem - it merely means the Standby Battery may suffer from inadequate charging.

[ The cap should still hold sufficient voltage, though if the M4 can suffer because its IGN voltage is much higher that its + (Standby Battery) voltage, a diode should be inserted from the cap to the + (Standby Battery) so that it can discharge to the + voltage.
And not that I understand why the relay is there - as I read it, it is on whenever the Standby Battery is connected (and with sufficient voltage) OR when the IGN is on - why not omit the relay entirely? ]

7. Thanks for taking the time read that. Ignore the majority of that drawing. Just the part of the capacitor and the diode on the ignition line was the part I was referencing.

The purpose is to make the IGN line a bit more stable. IMHO if the IGN line is that sensitive than it seems like a good idea to me.

8. Phew!

Yeah - just a diode to the cap which is at the IGN (sense) input.

PS - I didn't read fully - just skimmed for and updated diagram else comments that stood out (but my 3D glasses detected nothing LOL).

9. Argh terrible pun terrible!
Lucky I haven't been keeping up with this thread so closely!

10. Originally Posted by OldSpark
Phew!

Yeah - just a diode to the cap which is at the IGN (sense) input.

PS - I didn't read fully - just skimmed for and updated diagram else comments that stood out (but my 3D glasses detected nothing LOL).
Picked up the cap and diode today, will be installing it shortly.

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