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Solar powered battery charger for Nickel batteries

R

Raveninghorde

Jan 1, 1970
0
I am used to charging NiCd or NiMH cells however I haven't done it
from a solar panel before. I'm happy I can hold the solar panel near
the point of peak efficiency but I am concerned about charge
termination due to the variable power available from solar panels.

Normally for NiCd I would use negative delta V for termination with a
back up timer. For NiMH I would use a thermistor for delta T
termination. However I can see potential problems with solar as the
power source.

In principle I could lay out a 60W panel in Arizona and charge the
batteries without problems - until someone parks a truck and shades
the panel dropping the charge current.

So what is the best way to terminate charge on nickel based batteries
with a variable power source?
 
M

Martin Riddle

Jan 1, 1970
0
Raveninghorde said:
I am used to charging NiCd or NiMH cells however I haven't done it
from a solar panel before. I'm happy I can hold the solar panel near
the point of peak efficiency but I am concerned about charge
termination due to the variable power available from solar panels.

Normally for NiCd I would use negative delta V for termination with a
back up timer. For NiMH I would use a thermistor for delta T
termination. However I can see potential problems with solar as the
power source.

In principle I could lay out a 60W panel in Arizona and charge the
batteries without problems - until someone parks a truck and shades
the panel dropping the charge current.

So what is the best way to terminate charge on nickel based batteries
with a variable power source?

Terminate at 40c , if the batteries get that hot in the sun, you may
damage them during charging.

Cheers
 
K

Kevin McMurtrie

Jan 1, 1970
0
Raveninghorde said:
I am used to charging NiCd or NiMH cells however I haven't done it
from a solar panel before. I'm happy I can hold the solar panel near
the point of peak efficiency but I am concerned about charge
termination due to the variable power available from solar panels.

Normally for NiCd I would use negative delta V for termination with a
back up timer. For NiMH I would use a thermistor for delta T
termination. However I can see potential problems with solar as the
power source.

In principle I could lay out a 60W panel in Arizona and charge the
batteries without problems - until someone parks a truck and shades
the panel dropping the charge current.

So what is the best way to terminate charge on nickel based batteries
with a variable power source?

Keep a device on the battery that measures all current in and out. It
can estimate the charge power well enough to prevent wear.

LiFePO4 and Li-Po are other options. They'll give you the same power
rate, much better density, less self-discharge in AZ heat, and simpler
charging.
 
R

Raveninghorde

Jan 1, 1970
0
Keep a device on the battery that measures all current in and out. It
can estimate the charge power well enough to prevent wear.

LiFePO4 and Li-Po are other options. They'll give you the same power
rate, much better density, less self-discharge in AZ heat, and simpler
charging.

I don't get to measure current out of the battery, the state of charge
will be unknown when the battery is put on the charger.

Unfortunately the customer specifies Nickel based batteries.
 
K

Kevin McMurtrie

Jan 1, 1970
0
Raveninghorde said:
I don't get to measure current out of the battery, the state of charge
will be unknown when the battery is put on the charger.

Unfortunately the customer specifies Nickel based batteries.

Not many options left. I've never seen a process to determine NiMH
charge level without the use of a long and continuous high current. If
you look at all the graphs from manufacturers, you'll see that there are
no absolute values to use as triggers. They're all deltas and they're
only valid for a range of currents.

- Put a LiFePO4 in the solar charger and transfer that to the NiMH.

- Let the battery hit thermal cut-off and derate the life expectancy.

- Use proprietary fast-charge NiHM cells that have a gas pressure sensor.
 
R

Raveninghorde

Jan 1, 1970
0
---
So it looks like, without expressly acknowledging your earlier
boo-boo, you've admitted that your reference was bogus since a PV
lead-acid charger won't work for NiMH.

Unfortunately, and to your detriment, you still haven't addressed the
OP's query, which was, basically, "What do I do if the sun goes behind
a cloud while the battery is charging?"

Instead, you parrot an expensive "Interchangeable thermistor" scheme
to guard against overcharging, which the OP has already laid out, but
which has nothing to do with PV shading during charging.

The way I'd do it would be to completely disconnect the battery from
the charging circuitry when the array was shadowed, and then reconnect
it when the sun came out again.

Got a better idea?


I remember reading a paper, which I can't find, years ago. The rough
idea was you could use short discharge pulses between charge pulses
and the voltage depression during the discharge pulse was related to
the state of charge.

Whether the technique actually works in practice is another matter,
but if it works then it wouldn't depend on the charge current being
constant.
 
R

Raveninghorde

Jan 1, 1970
0
Hi, I have designed a solar battery charger for an autonomous system
much like the one you describe. It uses NiCad batteries.

The batteries used aren't much like typical AA Nicd's, they are ranged
from 45A·h to several hundred A·h's. As you have already guessed, in a
photovoltaic power system, the charging is made on a "opportunity
charging" scheme. You don't have control on how much power you can get
at a given time, the panels output power will change drastically on a
cloudy day. You can't expect to have anything like constant-current
charging, so dV/dt charge termination will give you problems, since
you can have negative dV/dt due to the panels decreasing its output
current.

AFAIK, there are three types of charge controllers, and all of them
rely on final battery voltage for charge termination.
- ON/OFF regulators: These have two threshold voltages, a higher one
which disconnects power to the battery and a lower threshold which
reconnects power. Much like a thermostat does. These are the oldest
and less sofisticated ones, but you can see that power disconnection
relies on battery voltage compared against these thresholds.
- PWM regulators: These have a final charge voltage setting, and do a
PWM on the transistors that control power from the panels to the
battery. They can gradually control how much power goes to the
battery. The way they work is, if the battery voltage is much lower
than the final voltage setting, they let 100% of the power to the
battery, as the battery voltage rises and it gets closer to the final
voltage setting, the regulator lowers the power delivered to the
battery. Normally, battery voltage will equal the final battery
voltage value.
- MPPT (Maximum Power Point Tracking) regulators: These regulators
have a DC/DC converter and can adjust the voltage seen by the panel
when connected in order to obtain maximum power from the panels. I
can't give you much detail about these types of regulators, since I
don't know them very well. But I can tell you that they are supposed
to give you the full rated power from your panels, which PWM or ON/OFF
regulators wont do.

Normally, the batteries used in these systems are Pb based ones,
mostly because they are cheaper. But if high temperature behaviour and
in some high reliability applications, NiCd's are used. If you take a
look at Saft's Sunica + range of batteries, you will see how these
batteries are oriented for photovoltaic applications. And if you dig a
little bit in the technical manual, you will see that they recommend a
final voltage-based charge termination. They even recommend different
voltage settings based on daily depth-of-discharge.

So the short answer is; If your application looks anything like this
one, use final voltage charge termination.

Cheers,
Pedro

Thanks.

I am trying a method based on Maxim App note 484.
http://www.maxim-ic.com/app-notes/index.mvp/id/484

I am going to use a comparator to give an interrupt to the PIC which
will disable the buck convertor.

This should keep the solar panel sitting close to it maximum power
point.

I like the pwm termination method of lowering the current as you get
close to the maximum battery voltage. It should be possible to get
over 90% charge without risking overcharging.
 
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