# Solar charger

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#### FlatButt

##### New Member
Hi,

I have purchased two solar chargers,
One is to keep the starting battery in my Mobility Van topped up.
The second is to charge a 12v SLA battery connected to my Vehicles Security Camera.
Both Solar Chargers do their respective tasks very well.

My question is:
Is it safe to leave a solar charger plugged in to the 12v socket that is attached to the Vehicles Starting Battery, whilst I am driving?

Some friends say I must unplug it (it will cook if charging system on van is back feeding it)
Others say it will be fine to drive with it plugged in.

My belief is that I can leave it plugged in whilst driving. (Correct me please if I am wrong)

#### MikeMl

##### Well-Known Member
98% probability that it is ok to leave it plugged in. Would have to see a schematic to be sure.

Slight chance that the charger was designed by a complete idiot, or is being marketed by the Chinese, in which case they may have left out the $0.03 anti-back-feed diode to make more profit... Last edited: #### FlatButt ##### New Member 98% probability that it is ok to leave it plugged in. Would have to see a schematic to be sure. Slight chance that the charger was designed by a complete idiot, or is being marketed by the Chinese, in which case they may have left out the$0.03 anti-back-feed diode to make more profit...
I have a couple of Shottky's laying around somewhere, so I stick one inline to be extra safe.

#### MikeMl

##### Well-Known Member
Even if you had a published schematic of the charger, you would have to open it up and trace out the "as-built" connections. I have recently opened up some Chinese-built switching power supplies that have wire-jumpers installed in the PCB where there are places for RFI suppressor inductors and capacitors (common-mode RFI chokes). Methinks it is a cost-reducing, greed-motivated conspiracy where they initially design-in the RFI filtering, build a few with the components to submit to RFI certification, but then flood the market with tens of thousands of the damn things where the RFI filtering was illegally left out of the shipped product to be sold by Home Depot and Amazon...

#### MikeMl

##### Well-Known Member
I have a couple of Shottky's laying around somewhere, so I stick one inline to be extra safe.
The forward-drop (~0.3V at low current) of a Schottky rectifier in-line will perturb the charger's voltage-sensing and likely result in a less-than-fully-charged battery.

#### spec

##### Well-Known Member
My question is:
Is it safe to leave a solar charger plugged in to the 12v socket that is attached to the Vehicles Starting Battery, whilst I am driving?
Hi FB,

By definition, solar chargers would not function if it could be 'back fed', so you have nothing to worry about.

spec

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#### dr pepper

##### Well-Known Member
Mike has a point.
The manufacturers would be the only ones that would really know, but I suspect they wouldn't be forthcoming.
I'd look on the web and see if anyone else is leaving the thing connected up.
I've used 12v 1a solar panels before on my boat, and had them permanently connected without any issues.

#### spec

##### Well-Known Member
Mike has a point.
The manufacturers would be the only ones that would really know, but I suspect they wouldn't be forthcoming.
I'd look on the web and see if anyone else is leaving the thing connected up.
I've used 12v 1a solar panels before on my boat, and had them permanently connected without any issues.
No, if the solar charger back fed it would not work because at night, for example, when there was no output from the solar charger the battery would discharge back through the solar charger.

As far as I know, all solar charging systems must have a reverse blocking mechanism of some kind.

spec

#### FlatButt

##### New Member
Had a chat with the Marine dealer that I bought them off and he say's tp leave them connected. As he has sold many to boaties etc, he feels that they will be fine. Apparently he has never had one come back. So I'll follow his advice and see what happens.

#### MikeMl

##### Well-Known Member
...
As far as I know, all solar charging systems must have a reverse blocking mechanism of some kind.
I have a Chinese one here that draws a couple of mA from the battery under charge when the panel is dark. If the panel was left disconnected for several weeks, it would discharge the battery.

#### spec

##### Well-Known Member
I have a Chinese one here that draws a couple of mA from the battery under charge when the panel is dark. If the panel was left disconnected for several weeks, it would discharge the battery.
Did you mean connected Mike?

Nothing is perfect in electronics, or life in general, Chinese or otherwise.

7weeks * 24 hours per day * 2mA= 336mA/hours, although undesirable, is hardly discharging the battery.

I haven't worked it out, because we do not know the A/H capacity of the battery, but that loss of charge is probably less than self-discharge of the lead/acid battery itself (5% per month). Assuming an 18A/H lead/acid battery, the self discharge over seven weeks would be 1.58 A/H, at 25 Deg C plate temperature. http://batteryuniversity.com/learn/article/elevating_self_discharge

I haven't measured the reverse leakage of modern automobile alternator rectifier diodes but, certainly, the earlier rectifier diodes were quite leaky, especially at elevated temperatures.

And Schottky diodes, in general, are quite leaky, especially high current types.

Anyway, the OP's main concerned is damage to the system while driving.

spec

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#### MikeMl

##### Well-Known Member
... If the panel was left disconnected for several weeks, it would discharge the battery.
Did you mean connected Mike?
I meant leaving the charger connected to the battery, but no panel connected to the charger. Scenario is a boat that I keep at Lake Powell. I once forgot to plug in the solar panel when storing the boat for a few months. When I returned, battery was stone dead. If the panel had been plugged in, the boat battery would have been maintained to >90% capacity. Before adding the solar charger, the battery would have been ~70% of capacity due to self discharge, still enough to start the V8. In a few months, the charger parasitic current added to the self-discharge of the battery to completely discharge the battery. I subsequently replaced the charger with one of my own design that had a parasitic leakage of a few uA.

I haven't measured the reverse leakage of modern automobile alternator rectifier diodes but, certainly, the earlier rectifier diodes were quite leaky, especially at elevated temperatures.
The old Ford pickup truck I leave at Lake Powell to launch the boat would have a dead battery after being parked for a few months. The parasitic current being drawn while parked was traced to the alternator, but not for the reason you stated. It turns out that the alternator was full of carbon dust that had worn off the brushes. Simply washing out the alternator with a high-pressure washer reduced the back-leakage through the alternator from ~30mA to a few uA.... I installed a battery disconnect switch to completely isolate the battery from the vehicle while parked.

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