diy solar

diy solar

Battery cables

Capt Ted

New Member
Joined
Oct 27, 2022
Messages
1
I am replacing AGM 24v bank with LifePo and going about it backwards. Recently bought the boat, converting to live aboard/ sailing and will strive to minimize power consumption; I do not yet know what that will be. Freezer, Fridge, fans for Diesel heaters, lights, fans, water pumps, macerator, computer, nav equipment, kitchen appliances, Tool battery chargers and large amp pull infrequent use of windlass. 12v starting batteries are on separate system. Presently, outfitted with 6 flexible solar panels, MPPT, twin 80 amp alternators with Wakespeed alternator regulators, and a programmable battery charger: AGM/ Lifepo. Four LifePo 24v, 200 AH batteries will arrive next week. I will wire each side of each battery to a bus bar with equal length cables and from bus bar to battery switch to fuse to Main bus. Present bank wired with 1 AWG (four 6v 150 AH batteries wired in series) between batteries and to house bank main.
LifePo Batteries each rated for 200A continuous discharge and charge current, my present max charge capability is about 135 amps via combination of both alternators and will be adding a wind gererator.
Question: Cable size? Cable run is 1' battery to bus, 1' bus to main. I see varying reccomendations: 1 AWG for less than 5' run, and yet using a 24v wire sizing calculator: 24v - 200A - 1' run requires a 3/0 AWG cable. What say you?
 

Attachments

  • tempImageEzmQiV.png
    tempImageEzmQiV.png
    2.5 MB · Views: 19
  • tempImageLuiw8C.png
    tempImageLuiw8C.png
    2.2 MB · Views: 18
24v - 200A - 1' run requires a 3/0 AWG cable. What say you?
Where did the 200A come from?

Battery cables are sized to handle either the max charge current or the max discharge current, whichever is greater.

It sounds like the charge will max out around 165A (135A plus a wild guess on my part for Solar and wind charge current).

Do you know what the max discharge rate will be?
Freezer, Fridge, fans for Diesel heaters, lights, fans, water pumps, macerator, computer, nav equipment, kitchen appliances, Tool battery chargers and large amp pull infrequent use of windlass.
Will any of this be AC? Will you have an inverter? How large?

The largest draw is likely to be your Windlass. Do you know what current it needs? Also, will the windlass ever run without the engine running? (if the engine is running a lot of the current for the windlass will come from the alternators.... not the batteries.

Do you have bow thrusters? (These are also very high current)

It is a bit of work, but I highly recommend doing an energy audit/survey to determine the needs. This spreadsheet is what I use:

Question: Cable size? Cable run is 1' battery to bus, 1' bus to main. I see varying reccomendations: 1 AWG for less than 5' run, and yet using a 24v wire sizing calculator: 24v - 200A - 1' run requires a 3/0 AWG cable. What say you?

Let's assume the 200A is the max current that the system will see and figure out what the wire size should be.
The fuse on the system needs to be 25% greater than the max continuous current. 200A x 1.25= 250A. The wire needs to be able to handle the full 250A. Your 4' round trip distance is short enough that we can pretty much ignore voltage drop so I recommend using this chart for determining wires size.

1667349519799.png

I recommend 90deg C marine-rated wire for ALL wiring on boats so you can use 2/0 unless it is in the engine compartment. (3/0 in the engine compartment)

Note: I assume you will want to run off of a single battery, consequently even the wires from the Bus-Bar to the battery need to be full-sized even though they will typically only carry 1/2 the current.
 
Last edited:
Back
Top