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I have a small electric vehicle that currently uses 4 x 12V 8Ah SLA batteries wired in series as its 48V battery packs. I have 2 of these packs.

In order to charge the batteries, I unplug all of them, plug them into a parallel wiring harness and use a 12V battery charger, either AC powered or solar panel with 12V charge controller. Then once charged, I have to re-wire them in series (the batteries have F2 terminals and the series/parallel wiring harnesses have female spade connectors)

This works but is obviously annoying.

I'm wondering what my options are. I currently see two possibilities:

  1. Continue this series/parallel song and dance, but use DPDT relays to easily switch between "charge" (parallel) and "run" (series) modes. Do you see any pitfalls with this idea? I really like the ubiquity of 12V chargers. I could have each of the two battery packs switched independently so one is charging while the other is being used.

  2. Use 48V chargers. I'm not even sure I can get a compact enough solar panel system to get 48V? I'd like to mount the panels on the vehicle itself, so probably not more than 150 or 200 watts of panels. Are there inexpensive charge controllers that can boost the voltage?

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  • \$\begingroup\$ Have you considered letting your batteries in a series configuration and using a boost converter to attain the needed 48V ? That would also allow you to charge the batteries using your solar panel while running. \$\endgroup\$ – Harnex Sep 11 at 6:52
  • \$\begingroup\$ My standard answer if money isn’t tight is to use four separate chargers and let the secondaries float and keep your series batteries. Will help significantly with balancing them too, but most expensive solution. What does your solar charger setup look like? \$\endgroup\$ – winny Sep 11 at 8:48
  • \$\begingroup\$ If charge time not overly crucial you can charge one battery at a time with one charger and 4 x DPST relays || Winny's solution of 4 floating chargers is good for mains operation - harder with direct PV panel use. You can get DCDC converters with floating output on eg ebay or aliexpress - isolated outputs are MUCH rarer than nonisolated. \$\endgroup\$ – Russell McMahon Sep 11 at 13:05
  • \$\begingroup\$ Any reason I shouldn't be charging all 4 in parallel using the same charger vs. 4 chargers or one at a time? It seems to have been working fine so far. \$\endgroup\$ – tlrobinson Sep 11 at 17:34
  • \$\begingroup\$ My current solar panel is a Renogy 100W 12V panel with 30A charge controller, but I'm considering getting one or two new 100 watt panels to mount directly on the vehicle, so I could get different voltage panels. It just seems like 48V panels are pretty rare / expensive compared to 12V panels. \$\endgroup\$ – tlrobinson Sep 11 at 17:38
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If charge time not overly crucial you can charge one battery at a time with one charger and 4 x DPST relays.

Use of 4 DPDT relays and a single 12V charger works but relays must handle operate current which may be >> charge current, requiring heavier relays.

Winny's solution of 4 floating chargers is good for mains operation - harder with direct PV panel use.

You can get DCDC converters with floating output on eg ebay or aliexpress - isolated outputs are MUCH rarer than nonisolated.

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  • \$\begingroup\$ Thanks. Can you point me to an example of a DC-DC converter like you're talking about? \$\endgroup\$ – tlrobinson Sep 11 at 17:41
  • \$\begingroup\$ Also these are the relays I was looking at using: amazon.com/URBEST-JQX-12F-General-Purpose-Automatic/dp/… \$\endgroup\$ – tlrobinson Sep 11 at 17:43
  • \$\begingroup\$ I can get them for about $4 each on Aliexpress. Although I just noticed the max contact voltage is 28V DC. Maybe close enough? \$\endgroup\$ – tlrobinson Sep 11 at 17:52
  • \$\begingroup\$ @tlrobinson If you need near 30A then a relay wit known provenance and a good brand "is wise". Voltage is more about not arcing or withstanding make/break arcing (mainly break). YMMV but relays I have bought via AliExpress had very questionable specs. This does not HAVE to be the case, but should be your default assumption. | Relays from very reputable suppliers generally meet specs. Those from others often enough don't. | Look at eg OMRON. Dislike the cost. Go from there ... :-). | Again - be sure name brand items are genuine. \$\endgroup\$ – Russell McMahon Sep 13 at 2:01

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