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!)