The Ideal Diode / Leisure battery switch, charging thingy

[Aethelric]

FETs are great at low currents but are resistive and this will limit the charging current. Dave

Huh ? If you think you need to charge a L/B at greater than 95A then this one won't be adequate! but you will need to generate an adequate Vgs I guess or use a P channel FET instead like this one which will handle 40Amps which might just be rugged enough.

Yes, the P channel has a RDSon of 0.06ohms. which means it has a higher volt drop than a diode at currents above around 16A. May be a good idea to put a FET and diode in parallel.

The objective is to stop the S/B discharging into the L/B so the diodes would need to be in the S/B line, and usually in both. So we are really talking about being able to handle the full output of the alternator e.g when raising the roof. It can manage around 90Amps.

The relay simply separates the batteries if the S/B voltage falls below what it would be on charge.

Dave

[teenmal]

hi,
i work for a company called Autosparks,and purchased this excellent relay kit:

http://www.autosparks.co.uk/product_inf ... ts_id=1709

for £15 or for £26 all in,you get the relay,terminals and all cabling required.it actually connects to the starter solenoid,not the alternator,so may be much more suitable for a bongo.
when fitting i actually found that my bongo already had a relay fitted,but was missing a fuse,so i ended up getting my money back on this relay kit,but i think it`s well worth considering.if youy phone up for advice,ask for paul,as he`s an auto electrics whizz,and say that jamie told you where to go.
thanks
jamie

He might tell me where to go

[g8dhe]

What about this one http://uk.farnell.com/international-rec ... dp/8648018

162 Amps and RDS is only 0.004Ohm. Surely that package can't handle 162A

If you read the spec sheet it can't;
" Calculated continuous current based on maximum allowable
junction temperature. Package limitation current is 75A."

[corblimey]

If you read the spec sheet it can't;

If that's akin to reading instructions then there's my downfall

I just had a quick look and the it says

The TO-220 package is universally preferred for all
automotive-commercial-industrial applications at power
dissipation levels to approximately 50 watts.

That imples ~ 4A @ 12v, that's much less than 75A.

It's got a very low RDS compared to many other FETs, I guess a similar device in a more suitable package would be more suitable.

[g8dhe]

You need to read the spec sheets with a lot of care however, especially with high current devices as many parameters are calculated using pulse currents rather than continous. For instance the Abs. Max.rating is 333 Watts, the Continous Source current is 202A ( which is where the note comes into play at 75A) looking at the charts the 75A limit appears under the Max.Case temperature grapth. I can't see 50W quoted anywhere in the parameters only in passinging in the write up which implies from P=I^2R that I could be √50/0.004 = 111A which is still over the package rating! Anyway with a Vds of only 40Volts it is likely to breakdown after the first voltage surge from using the starter.

I think I'll stick to Video and RF myself and use a relay for charging batteries!

[corblimey]

I assumed the 50w mentioned is not directly related to the device but to the capabilities of the TO-220 package hence it not appearing in the specs.

use a relay for charging batteries!

KISS...my sentiments entirely, works for me and many others.

[scanner]

I'd just buy one of Martin's excellent "willinton" kits and save all the hassle.

You buy one - you fit it - it works.

You can't much more KISS than that?

[mikeonb4c]

If you are like me g8dhe, its the challenge that drives you. Sometimes, an idea comes out the other end that makes very good sense. I'd be interested to hear how you get on and good luck

[Manny]

After ruining a leisure battery by letting it go completely flat a couple of times and then blowing the fuse by the current surge when it tried to recharge I decided to fit a voltage sensitive relay in the LB output to protect it by switching off when it goes below 12V. I used a 6.2V zener diode as the reference for a dual comparator IC and since this gave me a spare comparator I decided to use this for the voltage sensitive split charge relay. When I was designing the circuit I considered Fets and in the end decided for simplicity and cost and not have to worry about heat decipation relays were prefered. So now I have a plastic box with two relays. Four connections, the SB, LB input, LB output and earth. I have had it fitted on the Bongo now for nearly a year and it is working with no problems. No ruined LB or blown fuses!

Manny

[Aethelric]

After ruining a leisure battery by letting it go completely flat a couple of times and then blowing the fuse by the current surge when it tried to recharge I decided to fit a voltage sensitive relay in the LB output to protect it by switching off when it goes below 12V. I used a 6.2V zener diode as the reference for a dual comparator IC and since this gave me a spare comparator I decided to use this for the voltage sensitive split charge relay. When I was designing the circuit I considered Fets and in the end decided for simplicity and cost and not have to worry about heat decipation relays were prefered. So now I have a plastic box with two relays. Four connections, the SB, LB input, LB output and earth. I have had it fitted on the Bongo now for nearly a year and it is working with no problems. No ruined LB or blown fuses!

Manny

I considered doing something similar, but I ended up buying one due to lack of time. The only issue I see with it is the relay clicking in and out on a partially discharged L/B. The sequence is
1. S/B voltage builds up to pickup voltage, (I think this is around 13V)
2. The relay connects the L/B
3. The L/B pulls draws lots of current and pull the voltage down below the dropoff voltage. (I think this is around 12.5V)
4. The relay disconnects the L/B
5. back to 1.

It does happen - the wee green LED can flick off and on quite rapidly on start up if the L/B has been used much.. I guess this will reduce the relay life, but I have no idea by how much.

Dave

[John and Kim]

Almost forgot my own thread there..............

OK I think some of you have missed the point, the fact being I dont want to use a relay and I do want to come up with a more modern solution.

OK you can get big FET's and the one's that have been shown are tiny, how about this one!;

http://uk.farnell.com/ixys-semiconducto ... dp/3582073

It's not just the current handling as you can still PWM the FET and use a much smaller device.

My original question still remains unanswered as I need to know ALL the things the circuit MUST do.

Please no more ideas with relays, it may work but I already know that and I don't want to buy anyone else's solution and want to design my own.

The 'Device' will be inserted between the Main battery and the Leisure battery with only one wire in, one wire out and a ground tab.
It will sense the Main battery is at a voltage indicating it is near full and then connect the Main battery to the Leisure battery.
It will not allow reverse current (from Leisure to Main)
It must limit the through (charge) current to protect itself.
It must isolate the Leisure Battery from the Main battery when not being charged (i.e when engine is off)

So something like a Relay with a diode action but without the associated losses.

I think I have a solution!

I too prefer radio and video but this 'Simple' problem had always interested me.

I don't use (thermionic) valves anymore but do like them! and mechanics is what my motorbike has lots of and I love it and am replacing the starter solenoid with a FET and taking out most of the bulbs and replacing with LED's.

There must be a solution in silicon?

Have I missed anything??

JnK

[corblimey]

Well there's nothing wrong with re-engineering something if it interests you. I've spent hours and hours developing and building pic pwm regulated bike lights and yet there are loads of commercial alternatives. Thing is my lights are better and also it's interesting and fun (if you're a geek ).

To answer your original question; it seems you've covered all bases with regard to core functionality so why not add some bells and whistles. What about some user feedback features e.g. battery health (starter and leisure), remaining capacity, low batt warning, etc.

BTW if you're interested in lighting http://www.candlepowerfourms.com is a mine of information for geeks.

[g8dhe]

How are you going to prevent discharge back to the S/B thru the SD protection diode ? The Vsd is of the order of 1.5V so a heavy load like the AFT is going to bring the S/B voltage well down in comparison to the L/B, so the logic driving the FET for chraging will switch it off, but then the protection diode will go into forward conduction.
Might be worth considering using a Symmetrical GTO thyristor, to provide the reverse protection and the switching function, not sure that you can get them in the sort of voltages and currents you would need here as they are primarily heavy duty devices for "real" power switching!

[Aethelric]

Almost forgot my own thread there..............

OK I think some of you have missed the point, the fact being I dont want to use a relay and I do want to come up with a more modern solution.

OK you can get big FET's and the one's that have been shown are tiny, how about this one!;

http://uk.farnell.com/ixys-semiconducto ... dp/3582073

It's not just the current handling as you can still PWM the FET and use a much smaller device.

My original question still remains unanswered as I need to know ALL the things the circuit MUST do.

Please no more ideas with relays, it may work but I already know that and I don't want to buy anyone else's solution and want to design my own.

The 'Device' will be inserted between the Main battery and the Leisure battery with only one wire in, one wire out and a ground tab.
It will sense the Main battery is at a voltage indicating it is near full and then connect the Main battery to the Leisure battery.
It will not allow reverse current (from Leisure to Main)
It must limit the through (charge) current to protect itself.
It must isolate the Leisure Battery from the Main battery when not being charged (i.e when engine is off)

So something like a Relay with a diode action but without the associated losses.

I think I have a solution!

I too prefer radio and video but this 'Simple' problem had always interested me.

I don't use (thermionic) valves anymore but do like them! and mechanics is what my motorbike has lots of and I love it and am replacing the starter solenoid with a FET and taking out most of the bulbs and replacing with LED's.

There must be a solution in silicon?

Have I missed anything??

JnK

Well if you are going to all that effort, it would make sense to have the threshold voltage temperature dependant, sensing the main battery temperature. (Google battery voltage vs temperature)
Also consider what happens when the leisure battery is quite discharged. When your device switches on, the main battery voltage will dip. You could either pulse modulate the device, or operate it in linear mode to regulate the charging current.
An indicator of whether the battery is charging or not would be good, and if you use a LED, why not a tricolour one to give an indication of battery condition.

Interestingly, we are getting closer to being able to use AA batteries as leisure batteries Consider, AA batteries are 1.5V so we need 8 in series. Each manages about 2AH. Now you should not really discharge a lead acid battery below about 60%, so a 100AH battery has only 40AH useful charge. Lithium batteries can be fully discharged without damge. So we need 20 AA batteries in parallel - thats 160 batteries total, or around £160. It's getting there But the charging mechanism is completely different.

Good luck JnK, and I'll be really interested to see the results.

Dave

[dandywarhol]

You could then by 40 Lidl AA battery chargers and run a VERY long mains lead (length dependant on the distance to your destination) with 8 extension leads plugged into each other and keep them charged up - doesn't sound too difficult...............