Re: Battery charging with intermittant power source

On Jan 11, 12:03 pm, Jan Panteltje <pNaonStpealm...@xxxxxxxxx> wrote:
On a sunny day (Sun, 11 Jan 2009 10:54:15 -0800 (PST)) it happened
"m...@xxxxxxxxx" <m...@xxxxxxxxx> wrote in
<75712fdb-02ca-4042-89a3-affa2c576...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:

Any item you buy has implicit some liability insurance associated with
it. Thus if it causes major damage, you can sue the manufacturer. Of
course, this explains why all the wall warts comes from China. "Honest
judge, the ACME manufacturing company doesn't touch the AC power line.
Blame XXXX for that dangerous wall wart."
Hopefully you have a watchdog timer in your charger. You can't take
the risk that the PIC goes off in a loop and whacks the battery.

Sure PIC has a watchdog, but I still have to see the first PIC in a loop.

Also,
current sense using the power fet seems way too inaccurate.

It is actually quite accurate, there is an I calibration pot,
I have run it through the  current range, comparing the readout
with a multimeter.
You do not want .001 percent, 1 percent is good enough for this
sort of thing.
This topic was discussed here some time ago (last year?), and
Rds on versus temp and versus I drain is rather constant.

Generally most manufacturer provide an independent battery protection
scheme as a safety, i.e. independent of electronics. In nicad packs,
it could be a PEPI. Having designed charger chips, I know you need
multiple protection schemes.

There are fuses in both battery leads, and a thermal fuse as second protection
in the charger.
Plus current limit, plus voltage regulation.

While you may have considered the possible states of a healthy gel
cell, you also need to consider the state of a gel cell on it's last
legs. You don't want to charge a battery that is not healthy.

If it is short, then the thermal fuse will go.
If it is open then no current will flow.

You want to run that car on gel batteries?
Or just use the car to charge those?

I want to run accessories on the gel cells. It's a bummer to have your
notebook computer or whatever drain your car battery when you are in a
remote area. So with the car not running, the gel cells are the power
source. With the car running, the alternator is the power source. That
is mostly what battery isolators do, and they would be fine if the
auxiliary battery could take the alternator voltage. [The isolator
uses relays to prevent reverse power, i.e. the auxiliary battery does
not get used to start the car.] Gel cells cant take as much voltage as
"regular" car batteries since they don't vent.

I use a slightly lower charge voltage, no problem as I can normally
charge for 24 hours.
The manufacturer of these gel batteries recommend 14.5 to 14.9 V with
a maximum of 24 hours.

To make it more complicated, I'd like to not use switchers. I want to
be able to use ham radios and scanners from these gel cells, so I
don't want the RF hash from a switcher.

I would use a series PNP on a heatsink.
You may run into trouble with that when the sun is on the car for a long time.
Have you considered fuel cells?

So basically I need to provide
the float voltage to the gel cell from the alternator, i.e. high
current low dropout regulator with a bit of tempco compensation for
the gel cell. Reverse power needs to be prevented. Now this regulator
will see the load of the electronics, which could be high if a HF
transceiver were attached. Thus you wouldn't current limit this
regulator, but put a bidirectional current limit on the gel cell.

There are 2 circuits, the charge circuit, and the low dropout load regulator.
There should always be a current limit.
The nice thing about PICs, with all those analog inputs, is that
they can monitor everything, and then switch off based on time.
In current limit or foldback for 2 seconds? Power down.

Probably something like a polyswitch would do the trick, though they
have really wide specs.

I like to refer as much as possible to software, it is easier to change.
The fuse is the last resort if all transistors short and nothing works.
Polyfuse is only for low power stuff I think?

What I do now is for long periods in the boonies, I bring 5 gel cells
that I charge prior to the trip. For charging, I use an off the shelf
3 state charger since I don't care about switching noise in my garage.
These are 50 to 70 AHr batteries picked up on the surplus market (data
center pulls). When a battery reaches 10V, it gets retired and I use
the next battery.

250 Ah is a lot.
My transceiver uses about 20 A at max power, 5 to 6 A at low power,
that would give you more then 10 hours transmit time at max power.
It is a Ranger RCI-2970DX:
 http://www.pepperelectronics.com/acatalog/RCI-2970DX-sm.jpg
 http://www.cbtricks.com/radios/rci/rci_2970/index.htm

Here is an ASCII diagram of a simple low drop out linear regulator I published
here some time ago:

                                                      PNP power              
+10 to + 35 V ----  a diode k ------------------------e   c---------------- + 8.3
                               |   |                    b T1    |      |
                              ===  |                    |       |      |    
                         10u  ---  |                    |       |      R3 5k9
                         tant. |   |                    |       |      |
                              ///  |     about +2.4     c       c      |
                                 LM317L-------------- b           b----|
                                   |    |      |        e       e      |
                                   |    R1 150 |        | T2    | T3   |  
                                   |    |      |        |_______|      R4 2k4
                                   |----       |            |          |
                                   |          === 10u       R2        ///
                                   R2 150     --- tant.     | 150
                                   |           |           ///
                                  ///         ///         2 x NPN  

For more current you need some extra transistor driving the base of T1.
R2 set the current limit, set by the beta of T1 x I R2.

I've done the PNP pass in chips. [It's a common scheme since PNPs can
generally handle more voltage than fets on a chip. Very common on
switchers that bootstrap their power to the control circuitry.] ] I'd
be more inclined to use a Pfet (probably two back to back to get
around reverse power). The PNP isn't great for low drop out. You have
to add anti-saturation circuitry. The efficiency is poor near
saturation since the base current goes up.

I don't trust software. Too many bugs.
.

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