diode for backcurrent protection

Have read the diodes page and I have some 1N4000 series diodes for various uses. What I am having trouble with is defining the type of diode to stop current flowing back from a battery to a permanently connected charger.    I cobbled a charger from an old wall mount/plug in 500mA 12v DC power supply and mounted it under the hood of one of my land rover discoveries that isnt used much.In the hope that I could leave on charge occasionaly to keep the battery ready. I should really take the time to mend the vehicle so it doesnt need this,I know,but I have my hands full as it is. I have looked around and cannot find a straight answer on what diode type to use to keep the current flow one way.The solar power sites say shottkys have a bigger backcurrent,the split charge sites (and you know what I think of them) say shottkys are the one,no question. I am,as ever,confused.

I should say that,yes, I have read about forward voltage etc but would like to just be told "this type",by someone who knows.

Oh geez - you should see the, er, ... discussion I am having on mp3car about Schottky isolators for car dual-battery set ups. Did you know that relays were never designed to isolate batteries? And that relays are older than diodes? And that relays waste too much power with their energisation? No - neither did I. Nor does the majority of the battery-isolator industry nor UPS industry etc. And OMG - the 3W wasted in a relay! Or 13W for a 250A or 25W for a 400A relay! Can't compare that to the 0.3V x Amps at low Schottky loads, or 0.6V x 250A = 150 Watts of heat at higher 250A! And their voltage drop of 0.3 - 1.0V must be negligible compared to a typical high-current relay's 5mV or 50mV voltage drop! And at over $250, what a bargain compared to a pathetic $3 to $40 relay that will only last from .1 to 1 million operations (ie, 10 years plus?). Of course everyone knows that you don't have to isolate parallel batteries anyhow - unless perhaps they are getting old! Sorry, this OP's question was....? In short, you should not need a diode. But that is based on: - never leave a battery connected long term to a cheap charger. It will most likely cause the battery to overcharge (gas). - a charger should not need a diode to protect itself from the battery. Most small/cheap charges are essentially unregulated, ie, open-circuit they will often be 15V or higher, and normal battery charging should not exceed ~14.4V long term. Though a traditional or Schottky diode will have (say) a 0.6V or 0.3V drop, that doesn't necessarily stop gassing. And for regulated or lower-voltage chargers, it means the battery may be undercharging. The diode will stop the battery discharging into the charger's output lamps or LEDs or dummy load - if it has them and they are NOT diode isolated for the same reason. (They could be behind diodes with the charger sensing the actual output - ie, after that diode's voltage drop.) Those solar sites.... Bigger backcurrent - all the more reason NOT to use Schottkys, but that's where backcurrent means leakage current. (So many sites are full of people that think they know, else people pushing certain products (that can only be pushed onto the unsuspecting). LOL!) IMO they are talking about PIV - Peak Inverse Voltage - the reverse voltage a diode can withstand before breaking down. And any 1N400x will handle that. (It took a while for Schottkys to have anywhere near the PIVs and forward currents that "the old Silicon" diodes had!) The smallest is the 1N4001 with 50V PIV. Most common is the 1N4004 with 400V (the usual recommended for relay snubbers (the coil's voltage spike suppression - typically ~100V, sometimes ~200V or more)). There is also the common 1N4007 (1,00V - 1kV PIV). But you only need a PIV of about 12V; say 20V for some margin, ie 50V or more with a 1N400x, since it is in series with the charger and battery. (And any voltage spike is likely to be minimal.) All 1N400x diodes have a forward current rating of 1A. Even my 6V/12V 2A/8A charger exceeds 14.4V. A few night back I finally topped up my old 12V wets and my spare 12V AGM. In theory, I check every few hours, switch down to 2A when they hit ~14.4V, and then turn off when 2A hits 14.4V. In practice, I forgot the AGM until the next morning when it was ~15.6V! That's when I should have had it through a mains (wall plug) timer device - but I never forget things like that LOL!      Now charging my 38AH AGM with a charger like that is stupid. The battery's replacement cost (retail) is $660! (Yet the 100AH and 110AH versions are cheaper - ~$500 each.) But you'll have to decide your economics for your charger versus battery cost. Next time Addi have their SMPS powered 8A "intelligent" charger for $25, I'm buying two! They have 4-stage charging (3-stage for AGM) and will automatically switch down to a safe float voltage (typ 13.3-13.8V) when the battery is full. Cteks are a great similar charger, but cost well over $200 here. I have read of people getting Cteks for $40 upwards stateside. But IMO in your case - no diodes - you need a voltmeter (DMM) and to watch the battery's voltage instead. The only similar thing I have done was for a motorcycle battery with a supposed charge limit of 0.5A. I used my older 1A trickle-charger with a resistor to limit the current.    Does that help? If you want to use the diode, then the IN400x should be fine. In fact better if your charger's Open Circuit voltage is above 15V (ie, 14.4V + 0.6V diode = 15.V). Maybe a Schottky if the charger's OC voltage is under ~14.7V. And if 14.4V or lower, then no diode... But hopefully you understand my verbiage above. (Or, hopefully I haven't written in too much haste yet again!) Apologies for my opening digression, but for some pathetic reason, that probably helped me. (LOL!)

Slightly off topic Peter, I charged my 8 year old car battery off of a regulated 35amp power supply fixed at 13.8 volts, the ammeter was showing 10amps draw! That was 3 weeks ago and it's never performed better!

I was going to say that you aren't off topic, and and that you merely confirmed what I wrote..... But that reply was on the aforementioned Schottky isolator thread (on another forum) that I just posted this morning. I described the normal battery charging of (max) 14.4V etc, EXCEPT for battery maintenance which involves >15V to equalise cells, and often charge currents of AT LEAST 20A to blow away the effects of sulfation after long storage or the flattening of car batteries. In those cases, a BIG current may be the only way to recover the battery. The high current blasts the sulfation of the plates. That's probably what Howie did. (Probably? Why don't I just commit to it... "Without doubt"!)    Not that Howie and I ever compliment each other (perish the thought! LOL), however we do seem to complement each other. This has been yet another example. But how did Howie know what I just wrote on another forum? Honestly, his abilities and perception freak me out!      BTW - that other forum thread seems to have taken a turn for the better. First is a willingness to "give it a go" IMO admirable in any situation, provided it is based on sound information (yeah, MINE!). Second it that I think some issues are being understood...

What amazes me is that we do it through a 10 hour time delay as well

I forgot that! IOW you know what I'm going post before I post it! That explains it then - I must be sensing your thoughts. I knew I couldn't be capable of pre-thinking something that you understood...

I thought that I had posted a long reply last night but its not showing now,so......thanks Spark,again, thats the stuff I needed to know. The "charger" in question is an old plug into the wall 12vdc power supply that I had laying around.I havent measured what it puts out but I plan to soon.And then run it through a timer plug.Its just to keep my No2 truck ready for use whilst I concentrate on my No1 truck.I will fit a hardwired voltmeter as soon as I find a digi one that fits in the hole where the useless clock usually sits. re smart/intelligent chargers....I bought c teks cheapest 8 stager from RoadPro for 70GBP delivered and am very pleased with it.I plan to buy some of their 4 stagers to leave hardwired to my trucks and caravan,but if aldi come up with some first,well....Oh,RoadPro will sell you some nice diode based battery management systems,since you are such a fan ! shirker PS.    had a look at the forum you mentioned - boy,you been busy ! Making friends and influencing people,huh? Oh,and I appreciated your prologue so no apologies needed

Ha - more ESP - synchronous posting! Yes, that other forum thread is interesting. If only people answered simple questions... In lieu of answers I tried a few truths - like alternators do not stop charging when a battery is full etc etc. Surely people realise it must still "charge" for the engine etc, for the other battery, plus the main battery's float current. Nope - people drive around with "nothing on" etc. All I'm given is the paraphrasing of simple definitions.... But I understand the complexities, especially when people think that advertising blurb refers to all cases. EG - Schottky diodes have ~half the voltage drop of older isolator. Yes Dear, they have ~half the voltage drop of tradition silicon diodes - say 0.3V instead of 0.6V at low currents, and usually 600mV to 1.0V at high currents (250-400A). IMO that is significant, and somewhat more than the ~50mV of equivalent relays! But really - diodes! Not for the masses. This is my 3rd "discussion" in ~ 3 years on the forum. The first dude eventually took his bat and ball (he deleted all his self-incriminating posts. Poor boy - he thought he knew it all with his 20 years of experience in the industry. LOSER!) But the 2nd finally understood. I was going to leave this 3rd, but I think it is a special case, and a recent reply was promising.

Hmmm, my first "legal" (ahem!) car was a volvo 145 that had a habit of blowing alternators and regulators (seperate units back then) with such regularity that I drove around with 3 batteries and a charger - that would get me where I was going (without alternator) and when I got there I could charge enough batteries to get me home ! So much for driving around without alternators.There must be some serious vested interests in these diode battery managers cos they are pushed hard,all over the place. As for the other forum, ride em, cowboy ! Yeehaw,etc etc